Despite some glitches & technical issues, Mass Effect's story alone is worth the full price of a 360 game.
If you though the story was complicated, the combat will mess you up. Its not bad at all, but if you're expecting a run and gun action, then this ain't for you. The combat is basically like KOTOR and Gears of war in a blender. It's gun and cover action with varied ability options and skill upgrades. Unfortunately, depending on the class you choose for Shepard, combat can be a pain in the ass or a very fun experience. Be sure to switch to casual mode when it gets tough. In many places, you may get stuck which can be another pain.
Visually, Mass Effect is amazing at a technical and artistic standpoint. Characters show emotion and move with fluid animation. Details on water and surfaces are realistic. The default male Shepard looks great but the customs models that you can create will likey end up looking freaky. Its not the best character customization (that honor goes to fight night Round 3) but you'll really have to put some time into it to make it look decent. Another problem is that graphical pop ups happen a lot which may take you out of the experience. Unreal engine 3 does a great job but maybe Bioware could have made a proprietary engine instead as many of todays games are using UE3.
The dialogue is what sells this game. Top notch voice acting plus different kinds of choices for dialogue will make you want to believe that all of this is actually happening. My favorite were Seth Green who plays Joker, a concieted, smartmouthed, outspoken pilot of the Normandy and Kieth Davids who plays Captian Anderson, a professional captian who really gets into your heart once the story plays out.
Overall, Mass Effect is a great game that gamers should look into no matter what the problems it has.