So far the majority of what I see (correct me if I'm wrong) are some great in-depth reviews of specific Fairy-types such as Mawile and Togekiss. I think we can all agree that there are a good few useful Fairy-types to choose from. But how does this in turn affect the rest of the metagame? To start things off, here is an interesting set I've been putting to use:
Garchomp @ Roseli Berry
Ability: Rough Skin
EVs: 148 HP / 252 Atk / 4 Def / 4 SDef / 100 Spd
Adamant Nature
- Earthquake
- Dragon Claw
- Iron Head
- Protect
The idea of the set is simple: lure out and eliminate troublesome Fairy-types while hitting anything else hard with powerful STAB attacks. The rest of the set details aren't really important to the point I'm making, so I won't bother explaining them.
Overall, I can say Fairy-types definitely have the largest effect on Dragon-types. Many Dragon-types now opt to use coverage for Fairies (i.e. Iron Head Garchomp as shown above, Flash Cannon Hydreigon, etc). As for everything else I have to say, Darkmalice has already touched on Fairy-types vs Dragon-types so I see no need to repeat what he said. Dark-types and Fighting-types seem to not be phazed very much compared to what I was expecting; they usually just run their typical sets and switch to an ally Steel-type or such when confronted by a Fairy-type. Bug-types are almost uncaring about Fairy-types, as Scizor and Volcarona (the most common Bug-types) can easily work around Fairies courtesy of their secondary typings. Some less common Bug-types like Heracross are definitely hurt by Fairy-types, though.
Countering Fairy-types in the metagame has a tendency to be a tad difficult, as common Fairy-types such as Mawile, Gardevoir, and Togekiss have optional methods for dealing heavy damage to Steel-types (Brick Break, Focus Blast, and Flamethrower, respectively). For the most part Gardevoir will fall to a Bullet Punch and Togekiss won't be using offensive sets, but Mawile in particular is a challenge to counter. Its solid bulk allows it to tank almost any unboosted Earthquake thrown at it (Mega Garchomp has only a 31.25% chance to OHKO 252 HP Mawile without a Sand Force boost), and common Fire-types like Talonflame, Chandelure, and Volcarona will take massive damage from Sucker Punch, making them pretty easy for a skilled player to work around. One option is to use bulkier Fire-types that can tank Sucker Punch, such as Blaziken, Infernape and Arcanine (Mega Charizard X also does a decent job). Arcanine in particular I have seen a fair bit, since it is a great way to handle Mawile thanks to Intimidate (It can also utilize Snarl as well, though I usually see offensive sets). Finally, there are also two notable Poison-types that can get past Mawile: Nidoqueen and Nidoking. With a Life Orb and Sheer Force boost, Nidoqueen (the less powerful offensively of the two) only needs 88 SpA EVs with a neutral nature to OHKO a 252 HP / 4 SpD Mawile 100% of the time with Earth Power. I definitely expect these two to see some usage for this reason, especially on hail teams where they can utilize Sheer Force boosted Blizzards.
So I guess if I were to conclude this, Fairy-types certainly have a large effect on the metagame, as they can fulfill everything from redirecting moves (Togekiss) to eliminating Fighting-, Dark-, and Dragon-types from the field (Mawile, Gardevoir) to setting Trick Room (Aromatisse) to great effect. Countering them is definitely a priority for any team that wants to see a lot of success.

Garchomp @ Roseli Berry
Ability: Rough Skin
EVs: 148 HP / 252 Atk / 4 Def / 4 SDef / 100 Spd
Adamant Nature
- Earthquake
- Dragon Claw
- Iron Head
- Protect
The idea of the set is simple: lure out and eliminate troublesome Fairy-types while hitting anything else hard with powerful STAB attacks. The rest of the set details aren't really important to the point I'm making, so I won't bother explaining them.
Overall, I can say Fairy-types definitely have the largest effect on Dragon-types. Many Dragon-types now opt to use coverage for Fairies (i.e. Iron Head Garchomp as shown above, Flash Cannon Hydreigon, etc). As for everything else I have to say, Darkmalice has already touched on Fairy-types vs Dragon-types so I see no need to repeat what he said. Dark-types and Fighting-types seem to not be phazed very much compared to what I was expecting; they usually just run their typical sets and switch to an ally Steel-type or such when confronted by a Fairy-type. Bug-types are almost uncaring about Fairy-types, as Scizor and Volcarona (the most common Bug-types) can easily work around Fairies courtesy of their secondary typings. Some less common Bug-types like Heracross are definitely hurt by Fairy-types, though.
Countering Fairy-types in the metagame has a tendency to be a tad difficult, as common Fairy-types such as Mawile, Gardevoir, and Togekiss have optional methods for dealing heavy damage to Steel-types (Brick Break, Focus Blast, and Flamethrower, respectively). For the most part Gardevoir will fall to a Bullet Punch and Togekiss won't be using offensive sets, but Mawile in particular is a challenge to counter. Its solid bulk allows it to tank almost any unboosted Earthquake thrown at it (Mega Garchomp has only a 31.25% chance to OHKO 252 HP Mawile without a Sand Force boost), and common Fire-types like Talonflame, Chandelure, and Volcarona will take massive damage from Sucker Punch, making them pretty easy for a skilled player to work around. One option is to use bulkier Fire-types that can tank Sucker Punch, such as Blaziken, Infernape and Arcanine (Mega Charizard X also does a decent job). Arcanine in particular I have seen a fair bit, since it is a great way to handle Mawile thanks to Intimidate (It can also utilize Snarl as well, though I usually see offensive sets). Finally, there are also two notable Poison-types that can get past Mawile: Nidoqueen and Nidoking. With a Life Orb and Sheer Force boost, Nidoqueen (the less powerful offensively of the two) only needs 88 SpA EVs with a neutral nature to OHKO a 252 HP / 4 SpD Mawile 100% of the time with Earth Power. I definitely expect these two to see some usage for this reason, especially on hail teams where they can utilize Sheer Force boosted Blizzards.
So I guess if I were to conclude this, Fairy-types certainly have a large effect on the metagame, as they can fulfill everything from redirecting moves (Togekiss) to eliminating Fighting-, Dark-, and Dragon-types from the field (Mawile, Gardevoir) to setting Trick Room (Aromatisse) to great effect. Countering them is definitely a priority for any team that wants to see a lot of success.