ME2 demo is pretty good, great in fact. Looks and runs like pc version, runs well, aside the occasional tearing. May be confusing for people who haven't played ME1, but there's a nice "Last time on Mass Effect" video. Before making your character and the intro. Don't remember if that was there on ME2 vanilla.
Either way, it's a damn beefy demo. It includes the entire intro segment, up until you gain Miranda and even a bonus part on Omega trying to recruit Mordin. Nice and lengthy.
Controls are weird for me because I'm too used to play ME with kbm now, but they're much better than ME1's on 360.