Oh you, sweet Aegislash! I have also tried the suspect test, and in my 10 games so far, only 2 did NOT have Aegis. I even saw a couple of good teams built around it. The popularity is rising. Even if my teams did not include it, I'm seriously considering it, as it can play quite a few roles, without having drawbacks.
Aegis is a unique tank that has at some point to turn into the offensive. The problem that I see is that if it gets a SD up, no psychic or ghost type is able to take a Shadow Sneak, which will limit at an extend the usage of such pokemon. One would argue that we found reliable checks to the Lati bros, M-Zam, M-Metagross, but it also stops all those pokemon that took a sudden rise from UU to OU. Examples include Victini, Starmie, Gallade (non and mega), Jirachi, and M-Diancie (who cannot do anything against it). It is good on one hand to have more diversity in the tier and stop monsters like the Latii, but it limits your choice of pokemon to use.
Now, in terms of what is resistant and can revenge kill it or simply check it.
M-Metagross has a chance to OHKO a 248HP / 252Def Aegis
Calcs here:
Mew can take the Shadow Sneak, burn it (even behind the King's shield) and proceed to stall it to death.
Out of non-megas, Garchomp, Bisharp and Excadrill can all destroy it, given they lose a 30-ish % of their health first, which is not that much. Bisharp might have a bit harder time against it, as a badly timed Knock Off, can half the killing potential, but it sure has less to care about defensively.
Megas that don't really care and can do hefty damage are M-Lopunny, M-Zards, M-Metagross, M-Sableye, M-Gyarados (probably the best counter to Aegis), M-Manectric, M-Tyranitar (provided it OHKOes), M-Scizor, and M-Venusaur (provided it has HP-fire).
As stated above, the pokemon that can check or even counter Aegis are mostly megas and select OU pokemon. This brings me to the assumption that people using Aegis will try first to get rid of their opponents mega, and then have a really easier time against them. The issue that one can have 1 mega per team limits the options, yet not so much. Do you see where this is going? People will start to teambuild having Aegis in mind. While it is generally encouraged to think of common threats when building a team, people will always try to include something specifically designed to counter Aegislash, and in my opinion this limits the choices people have, and that's a big problem for the metagame. Given that now with ORAS, we have a plethora of pokemon in OU, and as such many different types of threats, this issue might be limited (as it would not be crucial for the metagame).
Right now, I'm on the fence on the Aegis test, but I'll come back when I have more results in my hands.