Worth Buying/Playing with a few caveats. Not just for fans of the book(s).
The story to the game is nothing revolutionary: evil forces are spreading themselves for some evil purpose, led by an evil looking overlord and doing evil things. The player is in the role of Eragon, a young man who seems to be an orphan (the game is not entirely clear in this aspect. Perhaps the book is, but I have not read it) who is destined for greater things. Unfortunately, there are quite a few loose ends left by the end of the game, with many characters seeming to exist solely to get Eragon to another town. I would assume that this is in order to get everyone to buy the sequel, should it be made, but some things that should be wrapped up in this particular chapter are introduced and then never addressed again. But a somewhat weak story (one must remember that this game is based on a book written by a 15 year old) can be forgiven if the gameplay is engaging enough, right? In Eragon's case, this is mostly true. Combat is fluid, complete with strafing ability and unlockable combos, the spell system is great fun (I experienced no real lack of responsiveness from the touchscreen outside of one spell), and upgrading is satisfying without being tedious. It starts off as a bit ungainly, but after a few tries it becomes less of a problem to remove the stylus to draw a spell or enable the bow. In the case of the spells, I found it very satisfying to "flick" the icon in the direction of the enemy and watch them react. And although the bow has been termed "useless" by some reviewers, with a little patience to upgrade it, it becomes on of the more powerful weapons and helpful tools. There are enough "recharge items" (for health, spell power, and arrows) scattered around so that you don't have to be stingy with anything or be overly worried of dying, unless you plunge yourself into battle without a plan. The environments are well designed and have enough texture and variety to keep the world interesting to explore.
There are, of course, a few drawbacks. The first and largest is the length of the game: it just feels anti-climactic at the end. This was the first and only time I have been disapointed to defeat the final boss in one battle (something I would normally welcome with tears of joy). However, most games on the DS so far have been fairly short, so I don't believe that one can fault this game in particular too much. It would seem just to be a technical limitation of the system (I hope I'm wrong on this and it's just laziness on the part of the developers). The sound in the game is fine, although lacking in the music department. Whether or not it is true, it seemed that there were two songs total in the game. If it IS true, than the developers should be criticized for their laziness. If it is NOT true, the composer should be sacked for his lack of imagination. There are occasional glitches in the graphics, but for the most part they are few and far between. There are several side quests included, but with the reward being most usually some concept art (pretty, but not exciting), there is little motivation to complete them all.
All in all, I would recomend that anyone pick up this game for a fun adventure, as long as they were prepared for it to be a short one. I can only hope that, if a sequel is made, there is a bit more length packed in.