I finished Mass Effect, at a few minutes shy of thirty hours. I imagine that if you don't try to finish every single little side quest in the game it's probably more like fifteen. The main storyline is where it's at; most of the side quests are pretty forgettable. It's not that they're not fun, but they're pretty repetitive, and their storylines usually don't tie in with anything else. Some of them are really well-done, and some are just weirdly abrupt and pointless. There's some definite fun to be had exploring uncharted worlds, but that gets pretty repetitive (and occasionally frustrating, given the vehicle controls) after you've done it a few times. Were I to play again, I don't think I'd bother with most of the side quests; without them, I don't think that the game would have felt repetitive at all. All of the main quest areas are very well-done, but the side quests reuse a LOT of architecture—there are like three building types for all of the different possible side quests, reused again and again and again. For as well-constructed as the rest of the game is, that's pretty puzzling to me.
But anyway, the core game itself is really great. The storyline and presentation are top-notch, and it's got a nice epic feeling and some fun plot twists. It's still in the vein of Neverwinter Nights and Knights of the Old Republic, but more of a departure in that the combat requires aiming instead of die rolls, and they were free to develop their own combat system instead of D&D. You don't get a lightsaber, but you can get a sniper rifle, a worthy but less pleasing substitute. All of the typical RPG stuff—killing monsters for shiny treasure—is in there, and while it's a space theme instead of the usual fantasy stuff, the world that they've created is really rich and full.
I played it on the PC, and I don't think it would have been nearly as fun on the console. Seeing it on the big screen and hearing it on the big speakers would have been a treat, but while it's an RPG, it still requires some FPS-like reactions, and my hatred of using a controller to aim a firearm is the stuff of legends. I can't even imagine how impossible the final battle would have been with a stupid little thumbstick.
Really, I can recommend it to anyone who likes RPGs, and if you liked Neverwinter Nights or Knights of the Old Republic, the chances that you'll love Mass Effect are very high, especially if you don't despise first person shooters. (I can't speak for the Xbox version, but if you like RPGs and console FPS games, I don't see why you wouldn't love it too.) It's a great experience. I'm looking forward to the inevitable sequel, though to the authors' credit, this game comes to a very satisfying and final conclusion.
Currently listening: Death Cab for Cutie—Your New Twin Sized Bed