I haven't said much while playing in the test, but here's my view of it (it came out a little long):
For the purpose of this post, Garchomp has a set of Swords Dance/Dragon Claw/Earthquake/Fire Fang with a Jolly nature and is holding a Yache Berry.
Stealth Rocks are up on both sides of the field. The player with Garchomp on the field is referred to as the offensive player.
The situation is that the player brings in Garchomp on something it forces out, either after a kill or on a predicted Thunder Wave, ect. At this point, the defensive player can switch in any of the Pokemon listed below to either outspeed or take and hit and attack back as Garchomp Swords Dances. This is not a very uncommon situation in the course of a typical battle, as Garchomp can threaten a good amount of common OU Pokemon. After the switch, Garchomp is attacked as it hits back with its most effective move. There can be no advantage gained by the defensive player switching at this point, as that would allow Garchomp a 'free turn' which it would not otherwise get. Each of the pokemon below gets one attack, and their most effective and commonly used options are listed below:
Scizor, Choice Band: Bullet Punch(47.21-55.59)
Latias, Life Orb: Draco Meteor(OHKO), Dragon Pulse(OHKO)
Heatran, Choice Scarf: Dragon Pulse(69.27-81.56)
Infernape, Life Orb: Close Combat(63.69-74.86) w/neutral nature
Lucario, Life Orb: ExtremeSpeed(38.37% max damage)
Gengar, Choice Specs: HP-Ice(63.69-75.42)
Swampert, Leftovers: Ice Beam(37.99-45.25), Avalanche(54.75-64.80) w/min attacking stat
Starmie, Life Orb: Ice Beam(72.07-84.92)
Azelf, Life Orb: Psychic(59.78-70.67), Explosion(OHKO)
Skarmory, N/A: Whirlwind(N/A)
Flygon, Choice Scarf: Outrage(OHKO) w/neutral nature
Jolteon, Choice Specs: HP-Ice(71.51-84.36)
Suicune, Leftovers: Ice Beam(40.22,47.49) w/min sp.attack
Bronzong, Leftovers: Gyro Ball(41.06-48.6), Explosion(90.5-106.7) w/min attack
Weavile, Choice Band: Ice Shard(56.98-67.6), Ice Punch(OHKO)
Aerodactyl, Focus Sash: Ice Fang(34.64-40.78) w/252ev, neutral nature
Mamoswine, Choice Band: Ice Shard(60.89-72.07)
Cresselia, Leftovers: Ice Beam(46.37-54.75) w/252ev, neutral nature
Crobat, Life Orb: Brave Bird(60.61-71.51)
Alakazam, Choice Specs: Psychic(79.61-93.85), 2.56% chance OHKO w/SR
Dugtrio, Choice Band: Earthquake(56.98-67.04)
Ambipom, Life Orb: Fake Out(34.36-40.5), Double Hit(59.22-70.4), Return(56.42-66.48)
All damage calculations assume 252evs and a positive nature unless noted. Attacks noted at a OHKO are regardless of entry hazards or any previous damage. Usage stats are from October on the standard ladder. Only pokemon in the top 60 in usage are listed.
Some other pokemon could use a Choice Scarf and/or uncommon move to hit Garchomp, but that will be addressed later. Of the above choices, four have a clean OHKO and two more will OHKO some of the time.
-Latias forces Garchomp out. It has two options to outspeed and OHKO Garchomp without harming itself. However, Latias cannot switch into a Dragon attack, and is not a very good initial switch in, although it is the closest to a counter that can be found.
-Azelf can explode to take out Garchomp. Each player loses one Pokemon, and this aspect of the battle is a draw.
-Choice Scarf Flygon can use Outrage to OHKO Garchomp. However, Flygon is in the same situation as Latias in that it cannot switch into a Dragon attack, but can outspeed and OHKO once it is in. The majority of Flygon already hold a Choice Scarf.
-Bronzong can explode for an 80% chance to OHKO Garchomp. This is a similar situation to Azelf. The chance of a critical hit or Fire Fang critical hit are small, but will happen from time to time.
-Choice Band Weavile can switch in and OHKO with Ice Punch. However, Weavile cannot switch into an attack itself, leading to another situation similar to Latias. About 1/4 of Weavile hold a Choice Band.
-Choice Specs Alakazam will be mentioned as it has a 2.56% chance to outspeed and OHKO Garchomp, however this percentage is so low that it should not be considered a reliable way to take out Garchomp before it can attack.
Skarmory presents a unique situation. It can switch in and Whirlwind Garchomp out, essentially putting off the situation for later. Skarmory will take a signifcant amount of damage while doing so, but will force Garchomp out barring a flinch or critical hit. This will result in the same situation occuring later in the match, although Garchomp will take entry hazard damage upon switching in again. If Stealth Rocks are the only hazard up, the only significant difference will be that Bronzong's Explosion is guarenteed the OHKO. This can be a viable stratagy on teams that employ additional entry hazards and are able to keep them on the field.
Ambipom is another unique situation. Its Fake Out allows it the chance to 2HKO Garchomp without being hit. There is about a 50% chance that Ambipom can do this with a combination of Fake Out followed by either Double Hit or Return. The chance is slightly higher for Double Hit, but this options presents the opportunity for a miss yielding no additional damage. Ambipom is also the least used Pokemon mentioned, and, being this uncommon on current OU teams, is not a large factor in taking down an opposing Garchomp.
Now, if the defensive player does not have one of the above options, they must use another Pokemon on the list to attack Garchomp for as much damage as possible. This attack will deal a varying amount of damage, but not enough to take out Garchomp. Therefore, it is imperative that the user use at least two of the above Pokemon with the listed moves/items. Once again, there may be other options, especially in choice of items, but these are the most common and powerful in the current OU metagame that accomplish this task. Upon seeing this second pokemon, Garchomp can switch out and come back in later without a harmful side effect, as it will have a reasonable amount of HP (40-50% is not uncommon) and will not take much damage on the next switch in. Garchomp's good defenses for a sweeper of it's power make this possible.
As many common OU pokemon are on this list, it is not a stretch that two of them be used. Upon closer glance though, Heatran must run a certain set, as must Gengar, Flygon and Weavile to provide the support needed. Other than Flygon, these Pokemon do not use these sets the majority of the time. Latias loses out on the sure OHKO if it uses either of it's two most common items, Leftovers and Choice Scarf, with Dragon Pulse. Aerodactyl is used almost exclusivly as a lead, so its ability to check Garchomp later in the game is questionable. Swampert must be at high health to check Garchomp, and could not be if used in the lead slot, which it frequently is.
This situation could arise at many points in the mid-game, so there is the possibility that one of your Garchomp checks has fainted and can't provide the support you need. Therefore, it is very reasonable to need to use three of these checks to stop Garchomp. Another reason to need three checks is Sand Veil. Now, the ability will not always be active, but it must be acknowledged. As most of these checks require that Garchomp be hit twice to take it down, Sand Veil makes it only a 64% chance that both of these attacks hit. Having a third check raises the chance to stopping Garchomp's sweep to 89.6%. With Sand Veil not always active, this represents a very reasonable chance to stop Garchomp.
For the reason of needing this third check to stop Garchomp from sweeping, I believe that Garchomp is Uber under the Offensive Characteristic. There is no other OU sweeper that currently requires three checks to ensure that it does not sweep. For example, Salamence, in the same situations, can be forced to Outrage and then taken out by a number of pokemon or taken out by priority regardless of what move it uses. It also does not have the luxary of being able to switch in a second time without any negative effects if it gets forced out. In some cases, Salamence can even be stopped by a single Pokemon such a defensive Swampert. Lucario does not need more than two checks in any case, as by choosing between Ice Punch, Crunch, and Thunder Punch, it looses coverage on key checks. With most Lucario opting for Cruch now, it has somewhat reliable counters in Gliscor and Gyarados. The counters to these OU sweepers may be weakened when they come out, just as Garchomp's may, but a second check alleviates many of the issues this poses. This is not so much the case for Garchomp.
For needing to choose three checks from a small list of possibilities, Garchomp fits the Offensive Characteristic. This forces drastic changes in a metagame where half of these checks are seen on less than 7% of current teams. In playing the suspect metagame, many of these checks are seeing huge jumps in usage. Skarmory, Bronzong, Scizor, Latias are all being used much more than in a Garchomp-less metagame, showing just how necessary it is to use these checks. Of the other checks, most have not fallen in usage. This shows the increased usage of Garchomp checks--and not by a small amount (Skarmory usage in particular has skyrocketed). The induction of a new, powerful Pokemon into OU should cause changes, but not the extent that Garchomp forces.