I went into this game not knowing anything about the franchise, or playing demon souls. After a couple of minutes of getting into the game, I was already dead on the ground at the feet of a 30 foot tall demon and I knew that this was going to be a tough one.
Dark souls reminds me a lot of ninja gaiden, the only other game of this genre that I've played (and that was years ago). But what made it a lot more satisfying was the fact that the entire world was interconnected. There are all kinds of shortcuts and interesting routes to take to get from one area to another, making this MUCH less linear than ninja gaiden was from what I remember. And the upgrade system was also a lot less linear, leading to many more options with much more replay value.
Having said this, I felt that towards the end of the game there were a handful of armor choices that were clearly superior, and my options felt a lot less diverse than they should have.
Luckily, this wasn't the case with weapons - they all seemed viable.
The boss fights are the real gems of this game, and there are quite a few of them. And unless you've already played the game or leveled yourself too high, you WILL die to them quite a few times before landing a finishing blow. I've had quite a few adrenaline rushes from the boss fights in this game, and found them to be the most memorable in my recent history.
The level up system was quite intricate, and included a few interesting stats (poise, bleed resist, roll speed, etc.) that made it satisfying to experiment. I think they could have improved even bit further by having characters improve their ability to use weapons/gear based on in-game usage of them, but this probably would have taken a lot of effort and I'm not blaming them for not including this.
The main negative about the game that was the weak storyline. I'm not sure why the game decided to keep new players in the dark (no pun intended) about so many things, but it felt as if I needed to go to online wikis one too many times to understand what was going on. There were a few reoccurring "oh yeah, that person" NPCs that I made the effort to talk to every time, but even after beating the game I don't really know who they were. They were just kind of... there.
Many games fill the gaps by having journals and cutscenes scattered around to discover, and Dark Souls had surprisingly few if any. While it's possible that I missed a lot of things by not "digging" enough, I counter by saying that the game is responsible for spoon feeding enough story to get the player interested in truly understanding what's going on. A game like KOTOR (2003) did a masterful job at this particular thing.
So overall, Dark Souls was a great experience that was definitely worth the $30. It has great replay value, very memorable boss fights, interesting gear, and I haven't even tried the PvP system yet. I found the story somewhat lacking, especially the ending, but to me this was a gameplay-driven game to begin with. It's a really good, really hard game that is virtually guaranteed to addict you until completion.