You can play Super Meat Boy at an expert level on it. :D
Correction. This one guy can play Super Meat Boy on it without too much trouble. His experience is not necessarily translatable to everyone, or anyone, else. We'll have to see when more people get the chance to try it out.
I'm not convinced that the controller will be a viable option for anything involving twitch reflexes. The examples he gave where rather simplistic. Meat Boy, while requiring twitch reflexes, does not have a complex control scheme (by design), and Spelunky, which kinda does, is not as fastpaced.
A Megaman X/Zero title or similar which do require both twitch reflexes and have a slightly more complex control scheme, I doubt the controller would suffice for. But if it does, cool. Until we know though, I remain skeptical.