Having wrapped up Fallout 3 until Pittsburgh, I'm trying out both Mass Effect and Oblivion. Both for a change of scenery(I'd have played STALKER, but Bioshock->Fallout3->STALKER would probably kill my interest in the setting forever...) and because I grabbed ME for a quick $20 and Fallout 3 convinced me that Oblivion deserved another chance!
Mass Effect: I hate the way your character runs, the guns-out-at-all-times thing bugs me... but I love everything else about the aesthetics! I'd kill for a fast-run option while on foot, similar to how everyone who ever played KoTOR spammed the +speed skill as soon as they got it..
I'm love-hate with the combat. Infiltrator. I absolutely adore the design of it and the use of tactics and such, and the Sniper Rifle wins my heart after Fallout 3's nonsense about not being hitscan based... but.. some of these fights are just fucking harsh, y'know? [spoiler]When that Krogan ambushes you when you save Tairi, holy fuck, that took me ten god damn tries. You got hit by the physics gun? You're gonna get gunned to zero health before you even begin the stand-up animation! The Colossus ambush before wasn't as bad, but I definitely died once because I was insta-gibbed as soon as the cutscene ended. Literally within two steps.[/spoiler] I love the thrill and challenge outside of those moments where you're killed so fast you're still hitting WASD.
And I have not gotten farther than that. Went back to main quest hub to fart around and do quests. Definitely a "in short bursts every day or two" game.
Oblivion: I've successfully modded out my greatest pet peeve in the game: character grunts made when fighting. Now maybe I can play the damn thing...