In my opinion, Rainbow is not really a feel-good type of manga, so if you're looking for a good hero story to lift your spirits Rainbow isn't the story for you.
But if that's not what you're looking for I would say that Rainbow does have some of the best story arcs I have read in a long time. It's drama, mixed in with redemption, and A LOT of tragedy. It has it's really depressing moments, but the promise of a good story providing a consistent stream of adrenaline-pumping moments keeps the reader reading.

Artistically its also a beautiful manga. I can't get over the awesome covers and consistency of the artwork through the whole series.

My only complaint is that the grungy tone doesn't change too much (hence my score of 8/10). As with most stories where there isn't much complexity in tone, it can get a bit boring.