Maybe the best game available in XBLA.
In Puzzle Quest you will create a character of your liking and start doing services for the queen. In time you will travel to far lands and get to know lots of interesting people and races like ogres, orcs, dwarves, elves and so on. In a nut shell your mission is to save the world from chaos and ultimate evil. Maybe the story in itself is not that unique, but the fact that this is a puzzle game with some real effort put into story is a wonderful thing. A few cutscenes and lots of text-based still images are available while you're making progress.
Graphics are colorful and enough detailed so you don't get the feeling that this game should have needed some HD upgrade or something. Sound is majestic and fully orchestrated, but some real voice acting is here as well. Unfortunately you'll soon notice that the same music and sound effects are recycled too often. To avoid getting irritated I recommend you to play in small doses.
Since this is an odd fantasy-puzzle-RPG you'll progress by completing missions and side quests like in any "real" role playing game. There are a lot of side quests and they'll grant you much needed experience points, but the basic gameplay is always the same in Puzzle Quest. You'll always do battles against an AI opponent and try to create rows of 3 in different pieces. If you have ever played Tetris then you'll immediately get the hang of things in Puzzle Quest. But still to master this game takes a lot of practice, but no worries: this is a long game so you'll have all the time you need. It'll take somewhere around 25 to 30 hours to play this game through. As I already said I strongly recommend you to play in small doses, since the actual gameplay is so repetitive.
Save system is just the way it should be: you'll get an autosave after every single battle and mission so there's no need to even think of saving your game. You'll always feel like your moving forward and gaining more experience.
There's also a multiplayer component either locally, via system link or through Xbox Live. The goal is to win a match just like in singleplayer mode. But since this is so heavily a story-driven game it's very likely that you don't want or need anyone to play Puzzle Quest with you: this is a one man adventure.
In the end, when taking the whole picture into inspection, I must say that maybe Puzzle Quest: Challenge of the Warlords is the best game available in XBLA.