Plain and simple, Mass Effect is one of the greatest games I have ever had the pleasure of playing. The story is amazing (you become totally immersed in it), the graphics are supreme (in fact they push the 360 a bit too hard at times) and the gameplay is signature BioWare .
If you haven't already picked it up, do yourself a favor and pick it up tonight. Be careful though, it's seriously addictive. I've spent 26hrs playing it thus far. I've spent a lot of time on secondary assignments to get my XP up for the primary storyline. I've heard you can finish the main storyline in about 30 if you ignore the secondary stuff, but what fun is that? :P