I'm currently level 116 on Demon's Souls, and I feel I've definitely played enough to give an accurate opinion on it. I'm a huge fan of JRPG's, but I also enjoy games like Fable and such. I honestly hated Oblivion, but I can't say enough good about Demon's Souls!
Some say the learning curve is steep, but I think that's the wrong way to explain it. There IS a little learning curve, but it's more like there's an experience curve. You learn the controls in the initial tutorial, and in my opinion that teaches you all the basic "battle info" you need to get going. There is, however, a HUGE frustration curve! I've lost 300,000 souls in just over an hour (lost 100+, then 50+ each time going back to try and retrieve previous "bloodstains). I consider myself to be pretty good at the game, although I am more of an "ask questions later" type of fighter, as I have been told by my friends I play the offense a bit heavily. The thing with this game is that you MUST learn how important fighting smart is (ie: attack, block, dodge, spell, items, ect...).
In my opinion, the controls, graphics, sounds, gameplay are all perfect. For some reason, each time I die and lose tons of souls, I still just want to keep playing. The game is JUST that addictive. The ONLY complaint I have with the game is the story. It's there, don't get me wrong, but there are little tidbits, like the broken archstone. The reason for the broken "land of the giants" stone is never explained, unless I just missed it. I wish Atlus would offer it up as downloadable content, but I'm not sure we'll ever see that. I accept the fact that some things aren't explained and such on the pure fact that this game is all about just being an amazing experience.
I honestly don't have the game right now. I let a friend borrow it, he played it for a couple hours, and I'm getting it back tomorrow, lol. This game isn't for everyone. If you're looking for a deep leveling system, amazing gameplay, replay value (I'm 55 hours in), and a big challenge, pick this game up. If you don't like these things, then stay away!