I bought one because I wanted a second CLS-controlled flash, and the SB800's were all gone. I like the SB900, but would have preferred the SB800 due to a lower price.
I do think the SB900 is better, though not more powerful most of the time. (Power increase comes from better zoom, and better matching of flash to DX/FX format). It's biggest benefit to me, in comparison with the SB800, is the ease of use of the controls. One wouldn't think a simple on/off/master/remote switch would make that much difference, but it's something that you deal with every time you use the flash. Not worth upgrading to get at the price you have to pay, but appreciated when you get one.
On the other hand, the flash is physically bigger (head, that is), and the storage case is significantly longer (because of how the diffuser and filter holder is stored). This also affects me about everytime I use the flash, because its a lot bigger to pack in the bag, and the separate zippered compartment for flash is a pain to have to open and close every time you use the diffuser, plus the filter holder nested in there is also fiddly to put away.
In use, they are about the same, except that the SB900 turns farther in one direction (useful).
If you have the opportunity to get an SB800 flash at the old price, you really have to wonder if it's worth paying so much more for the SB900.
