I suggest getting Demon's Souls solely because it might go out of print pretty fast like many Atlus published game.
I found Fallout 3 to be the better game though. Demon's Souls isn't all that great, imo. Out of the 8 levels that I played, I only like 3 and two of them can be beaten in 20 minutes or less. Boss battles are pretty disappointing. I like the boss scales and all, but fighting them is kind of boring - tower knight was the only one I found fun and challenging. Speaking of challenge, the game isn't as hard as the reviewers put it out to be; you just have to move slow and not rush in. Story isn't remotely interesting, and the combat system is just average.
Sorry for my small rant.
Finish the game, play on New Game +. That's where the real challenge begins.
The bosses you have been fighting are mini bosses of the levels. Fight the Archstone demons, it might change your opinion on boring boss battles.
The game IS hard ( unless you already watched the guides or videos on how to beat the levels or bosses.)