Historically, planeswalker cards are new to Magic, but they are now a dominant force in both constructed and limited. Due to the fact that they are mythic rares, they have a much bigger impact on constructed of course, but when you get one in limited, you play it and it's a game breaker when you draw it. Several different decks use planeswalkers in Standard and they have been used to powerful affect in both aggressive and control archetypes.

Why are they so good?

1. You get at least one use out of them when you resolve one. Play it, use it. Immediate return on your investment.
2. You get to keep using it for free. Sure, the first turn you had to invest a card and mana into it, but not future turns. Every turn, you can get an effect from the card without paying any mana or using other resources of yours.
3. They're surprisingly annoying to get rid of. They don't die to creature removal or even artifact/enchantment removal. With Oblivion Ring gone, there are extremely few constructed worthy cards that remove them. Sure you can try to kill them with burn or attackers, but that means you're spending some combination of mana, cards, damage and attack steps to get rid of something that they've already gotten a return from.
4. Against an aggressive deck, they're like Fog plus. Against a Control deck, they're often incredibly hard to get rid of, because they have so few damage sources.
5. They're flexible. Almost all of them have more than one powerful option to choose from when you activate them, even when you first play them.

The deck that uses the most planeswalkers is Blue/White Control. For example, here is one played by Chihoi at a MTGO Standard Premiere:

"Chihoi's UW Control"

• Planeswalkers (10)
• 2 Elspeth Tirel
• 2 Gideon Jura
• 2 Jace Beleren
• 3 Jace, the Mind Sculptor
• 1 Venser, the Sojourner

• Creatures (6)
• 2 Sun Titan
• 4 Wall of Omens

• Spells (18)
• 1 Cancel
• 4 Day of Judgment
• 1 Deprive
• 4 Journey to Nowhere
• 4 Mana Leak
• 4 Spreading Seas

• Lands (26)
• 1 Arid Mesa
• 4 Celestial Colonnade
• 4 Glacial Fortress
• 4 Island
• 3 Plains
• 1 Scalding Tarn
• 4 Seachrome Coast
• 1 Sejiri Refuge
• 4 Tectonic Edge

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He played with 10 planeswalkers cards in his deck and five different planeswalkers:

• Elspeth Tirel - Useful against aggression and control. Against aggro, you can either make more blockers or gain life, depending on the situation, both valuable against aggression. Sometimes you can even use its last ability to clear the board. Against control, you can create creatures to attack their planeswalkers and to defend yours. Also, the last ability is a great way to get rid of opposing planeswalkers.
• Gideon Jura - Incredible against aggression, often able to win games single-handedly. Can immediately be a maximum fog, which with 8 counters at that point, will often survive. Combined with the ability to destroy tapped creatures, can gut an aggressive deck, especially if you have decent blocking options. The additional ability to be a 6/6 for five mana that doesn't die to sorcery creature removal and is only vulnerable to instant removal at times of your choosing makes it strong against control as well.
• Jace Beleren - One of the drawbacks of other planeswalkers is that they're useless until you get at least four mana, but Beleren is a great investment for three mana. Against aggression, it's usually a cantrip fog, often both helping you get to Day of Judgment and to four mana. Against control, it can be nightmarish for them. Drawing six cards to every four of theirs is an overwhelming advantage in a control match up.
• Jace, the Mind Sculptor - The $90 super bomb for good reason. First, unlike most planeswalkers, it has four abilities, not three, all of which can be powerful if used correctly. The zero counter ability is so good, that it doesn't need any others. For a onetime cost of four mana, you get: to draw an extra card every turn, but not just any card! First, you can choose from any of your top three. Second, if you have other less useful cards in your hand, you can draw all three! If you have enough shuffling methods in your deck, you can maximize this madness turn after turn. It's insidious, because it's a hideously powerful effect and yet since it doesn't directly affect the board, your opponent doesn't always feel the necessary urgency to get rid of it. Even if they do get rid of it, they invested effort in getting rid of something that allowed you to search for a suitable replacement. For aggression, this is typically a must kill planeswalker, ignored at great peril. For control, the match up can often come down to who gets the first Mind Sculptor into play. I once wrote an entire article on the proper way to play with and against Fact or Fiction, this is one of those type cards. This card is so powerful, that in spite of its price tag and the fact that it's essentially a legend, most Blue decks will contain four copies of it.
• Venser, the Sojourner - Since Blue/White had so many good options in the Planeswalker department, why not give them their own exclusive one right? Sigh… Although all three abilities are powerful, only two commonly get used in a control deck, the +2 and the -8. Among the many powerful uses of the +2 ability is resetting other planeswalkers. In this deck, it's also great with Sun Titan, Wall of Omens, Spreading Seas and to a lesser extent, Sejiri Refuge. It also has subtler uses, like untapping a land for use on your opponent's turn or targeting your Journey to Nowhere when you are about to play Day of Judgment. It can also be useful for protecting your creatures before playing Day of Judgment. Unlike the other planeswalkers in Blue/White, it's very interactive with the other contents of your deck. If you aren't playing with permanents that are useful to exile, it's much less powerful. If you are playing lots of them however, it can be a game breaker very quickly.

Another standard deck that will often use more than one different Planeswalker (besides the ones I just discussed) is mono-green Elves. This is an example of one piloted by Daniel Sudyam to a first place finish at a game day in Louisiana:

"Daniel Sudyam's - Mono-Green Elves"

• Planeswalkers (3)
• 1 Garruk Wildspeaker
• 2 Nissa Revane

• Creatures (36)
• 4 Arbor Elf
• 4 Elvish Archdruid
• 3 Ezuri, Renegade Leader
• 4 Fauna Shaman
• 4 Joraga Treespeaker
• 4 Joraga Warcaller
• 4 Llanowar Elves
• 1 Molten-Tail Masticore
• 2 Nissa's Chosen
• 2 Tajuru Preserver
• 4 Vengevine

• Spells (4)
• 4 Eldrazi Monument

• Lands (19)
• 15 Forest
• 4 Oran-Rief, the Vastwood

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• Garruk Wildspeaker - All three abilities are powerful in this deck. Untapping land is probably the least exciting, because the deck already has pretty explosive mana, but the deck does have multiple creatures with abilities to sink mana into. Against both aggro and control, the ability to crank out 3/3s is powerful. The potential for several 3/3s for four mana is especially cool in a deck with cards like Eldrazi Monument that reward you for having multiple creatures in play. Wildspeaker also has a built-in way to reward this, because of its -4 ability.

• Nissa Revane - Another four mana planeswalker with three abilities that are powerful in this deck. Being able to put more 2/3 elves into play without necessarily paying mana for them or losing a card from your hand is good, especially while adding counters. Given this deck's ability to gets lots of elves into play, the life gain can be nuts. In the rare event you get to use the -7 ability, you win. The first two abilities are excellent against aggro and the first and third abilities are powerful against control.

The other planeswalker that sees lots of play in Standard is Koth of the Hammer, as in this RDW deck played by Sherwin Pu at the SCG Open in San Jose:

"Sherwin Pu's RDG"

• Planeswalkers (4)
• 4 Koth of the Hammer

• Creatures (16)
• 4 Goblin Guide
• 4 Kargan Dragonlord
• 4 Plated Geopede
• 4 Spikeshot Elder

• Spells (16)
• 3 Arc Trail
• 1 Basilisk Collar
• 4 Burst Lightning
• 4 Lightning Bolt
• 2 Mark of Mutiny
• 2 Searing Blaze

• Lands (24)
• 4 Arid Mesa
• 12 Mountain
• 4 Scalding Tarn
• 4 Teetering Peaks

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• Koth of the Hammer - One of the reasons mono-red can have a chance against control in the late game. The problem is sneaking it past Mana Leak of course, but if you do, the control player can be in big trouble. If turning Mountains into 4/4 haste creatures isn't exciting enough, what does control do if you get the emblem? When racing aggro, you get a 4/4 haste creature for four mana and they still need to take the trouble to kill Koth or face more problems. I've also seen Koth in Valakut decks, in part because of the mana producing ability.

There are 14 planeswalkers in Standard currently and the eight I mentioned are probably the most commonly used in this environment. That being said, I'm sure I've seen all of them tried in constructed at least once and I've seen Ajani Goldmane played several times. This frequency of usage in constructed alone should give you a pretty clear idea of the average power level of this relatively recent card type.

This should lead you to consider two important questions when building your next standard deck. First, what is my plan for dealing with opposing planeswalkers? Will I try to use permission, land destruction or hand destruction to prevent them from hitting play? Will I use massive aggression to either ignore them or batter them aside? The second important question is which planeswalkers are the right fit for my deck? Will my entire deck revolve around a planeswalker or planeswalkers? Which ones best compliment what my deck is trying to do? If I'm not playing with any, why not and have I lost my mind?