My brother bought this the other day, and I managed to play a good couple of hours when I was round his place.
To be honest I hadn't paid this game any attention at all, so I didn't really know what to expect. I was pretty much blown away, everything is just so well-done. I only did the first mission and spent a good while doing side-missions and exploring the citadel, and they really did a great job at creating a believable world with a ton of backstory to delve into. The graphics, soundtrack, voice acting, characters, conversations etc. are all top-notch.
I've been meaning to pick up a 360 for a good while now, and this game is the perfect excuse for me to finally get one.