Not bad, but not spectacular either.
Gameplay - If you played the first two games then you know what to expect. Run, gun, platform. On the upside, there are a lot of guns and each has multiple upgrades to make them stronger. The controls work perfectely and the health system has received a change for the better. On the other side, some of the weapons are worthless and the difficulty is a problem once again. The game starts off very easy and then gets really hard about halfway through. Also, Insomniac still hasn't learned that Checkpoints are a good thing and there are way too few Checkpoints in every level (read: about 2 or less). Though one major plus is that all the horrible racing and space fighting parts of the previous games have been nixed. Overall the game plays well, but has some major shortcomings that have been carried over from the first two games that bring it down.
Graphics - The graphics are excellent. Every character looks nicely detailed and the weapon effects look great. The worlds you visit look amazing as well.
Sound - The voice acting (as always) is terrific and the sound effects for the weapons and levels is top notch. The music is decent but forgettable.
Story - The story takes place after the second game but is set up so you don't need to have completed the previous game to understand it. The new villain in the game, Dr. Nefarious, is out to get rid of all organic life forms (or as he calls them "squishes") and make it so robots take over. Thus, it's up to the universes most unappreciated hero, Ratchet, to stop him with the help of his partner Clank. Overall the story is interesting but unfortunately the ending is extremely disappointing and doesn't serve much of a purpose except to give you a good laugh as it doesn't give much closure to the story. In short: the Jak series is much better in terms of story telling.
Replay - To be blunt, there isn't really very much replay to the game. After you beat it you probably won't want to go through it again. The game isn't very long either.
Overall Ratchet and Clank: Up Your Arsenal is a decent game but due some problems with the gameplay, would be best used as a game you rent over a four day weekend than as a purchase. However, if you were a fan of the first two, you'll probably like this one as well.