Solo does a good job keeping itself fresh, from what I've seen.
The environmental changes are much larger in scope than I'd expect, personally, and there have even been a few inventive boss strategies in there.
Also remember that the Quest is co-op able. Playing through it with my brother locally has been very nice, and this is one of few PS3 games I know of, that actually respects him playing as a seperate profile. (Is this rare on PS3?) So he earns his own characters, and has his own stats... VERY appreciated, as this doesn't happen on much bigger games, like Ratchet and Clank: All 4 One.
ANYway, it's keeping me intrested, and the few puzzles to earn special characters, mass diversity of items to throw, varied stage gimmicks, and introduction of more opponents is keeping things nice and fresh. I really like the ability to have a Co-Op campaign run, or VS, that adds quite a nice bump to it's overall value, to me.