Well I finished it for the first time last weekend and loved it so much that I've only been really playing Mass Effect 2 since Monday and I'm already 20 hours into it.
I'm loving Mass Effect 2 so much that I'm going to go back through Mass Effect again and play Paragon this time (since I'm renegade now) and see how some other decisions plan out.
My first full Mass Effect playthrough took me about 15 hours on easy, though now that I know more what's going on I could cut that down a lot. I didn't many side quests in that playthrough though. I'm going to read up on which ones affect Mass Effect 2 and do those if I can find a good list without Mass Effect 2 spoilers.