I think the Demon's Souls servers are still up. I can't check for sure because I don't have my PS3 with me at college, but based on some quick googling, it seems that they haven't been shut down yet.
Demon's Souls is hands down one of my favorite games of this generation. It is fantastically rewarding and addictive once you get over the initial learning curve. I really suggest reading about some of the mechanics on the wiki though, as someone else in this thread suggested. There are things like world tendencies and stat requirements for equipment that the game never really explains to you, but if you take the time to understand them it makes the game so much easier.
It is a LONG ASS game though. My first playthrough was about 60 hours. I realize most people won't be as addicted as I was, but that time was all concentrated in a 3 week period, so it didn't consume my whole life for very long.

I've been wanting to play Dark Souls for a long time now, and it's finally cheap enough that I might be able to get it.... might want to wait until after finals though.