I'm still only 13 hours into it, but so far it's right on track to becoming my favourite game ever - on any platform, since I started gaming back in the mid 80s on a C64.
Nostalgia still compels me to maintain that Planescape: Torment is the best RPG I've ever played (and the story and writing is certainly on a different level than ME2), but as an overall experience, ME2 is - so far - as good as it gets for me.
If it had been an entirely linear shooter with excellent dialogue and voice acting restricted to cut scenes, I might not consider it better than something like Uncharted 2 (ie. merely rather spectacular), but fortunately ME2 is vastly more open ended and complex than the third person shooters from which it has successfully (here in ME2, not so much in ME1) borrowed its combat system.
It's still not without flaws, but they aren't nearly large enough to detract from the experience in my opinion - unlike ME1 where the lacklustre shooter mechanics and the Mako exploration were rather glaring blemishes on a still great game.