Puzzle Quest 2 deviates in a good number of ways from its predecessor. Most notable are the changes to mana and that you now have a weapon. Overall I'd say it was a good change, as it doesn't feel like a recycled version of the first game at all, while still having the same basic gameplay.
With the mana changes, it's harder to become godly, but it also feels like every level gain is a really tiny improvement. Heavily investing in one stat has diminishing returns (which is fine from a balance perspecive). Enemies scale up a bit faster than you do, and once dead they stay dead, so there's a strange sort of progression going on there since you can't grind forever.
Similar to the first game, the encounter rate is through the roof. Fortunately you can skip a good number of encounters by simply ignoring the monsters. There's far less quests, but most of the quests in the first game were pretty pointless to begin with, so I guess this is can be seen as a good thing.
Fortunately the mini-games are toned down and varied, and luckily opening treasures has no failure. So it's nice to have freebies every now and then to break up the monotony.
The story, as best I can tell, bears no relation to the first game. Not that either one is special, anyhow.
If you like Puzzle Quest 1, I'd say this is a worthy addition. If match-three games aren't your thing, this game won't do anything to appease that feeling. I'll give this an 8.