I've played thirteen games against Grey Knights since they were released, against different gamers, both in friendly and tournament matches.
Of the thirteen games I have won 9 and tied 4. Not a single game lost.
In 5 games I used Blood Angels (4 victories, one tie); 2 games with Imperial Guard (both wins); 2 games with Necrons (one tie, one win); 3 games with Chaos (2 ties, one win); one with Nids (another win).
My impression is that they are tough as nails, but far from invencible. The hardest list to win was one formed by Castellan Crowe, 5 Razorbacks with assault guns and psy ammo, 3 dreadnoughts with twin heavy cannons with psy ammo and 5 purifier (are they called that in English?)squads. Of three games I managed to win one, and tied two.
The easiest wins were against the list that included the Paladins and Draigo with a Land Raider as transport. My opponent did sink so many points in just a single unit that my IG made short work of the rest of his army, and then beat the crap out of Draigo's Own.
I am not a particularly strong gamer, by the way. Neither my lists are very cheesy -I like to field units I fancy by their aestetics or background, not because they are specially powerful-.