Just played through half the demo. The first part of the demo is what I assume the first mission of the game. For more information on why you are where you are, you need to be aware of the Arrival DLC for ME2, which I just played the other night for the first time. Once I got to the 2nd part of the demo, in which you are fast-forwarded to a random mission somewhere in the middle of the game with some of your old squad, I quit. No need to spoil anything.
Game was fun, although it will take a couple hours to get used to the new interface. Just a lot of small changes that seem weird right now because I just played through ME2 again and these are a little different.
I guess the overall squad size for this game is going to be closer to ME1 than 2, which is a slight disappointment. Seeing as I never could play ME1, I really don't have that much of an attachment to any of them (bar Garrus and Tali). Also, since this game is supposed to be epic, I was hoping for a Normandy full of all the surviving squad members from 1 and 2, along with plenty of side missions for all. What was the point of saving everyone on my last playthrough if I can't use any of them this go around (my Paragon save lost Thane in the suicide mission...I thought he'd be good at crawling through a ventilation shaft!). Oh well, still pretty excited about this game, already pre-ordered it off Amazon with release day delivery.