If you're a full-time newsman, paparazzi or wedding guy, you know you want this flash. If you're a gadget hound who had to have the first GPS, iPhone and PDA on your block, get this SB-900. You'll love it and it works very well.
If you just want great photos, don't let me tell you not to get the SB-900, but I will tell you that you certainly don't need it. The SB-900 offers nothing significant over the SB-800, and isn't as easy to use as the SB-600. The other features are just fluff.
The only significant advantage of the SB-900 over the SB-600 and SB-400 is that the SB-900 (and SB-800) have a built-in white card to provide a catch light when bounced. I use a white Post-It Note on the other flashes to do the same thing.
Honestly, the only flash I ever carry is an SB-400, and my SB-600 is only used in my studio, set to manual, to trigger my pro system.
If you're a studio shooter, you'd be an idiot to get one of these SB-900s. I paid the same $500 for a complete three-head used Novatron system.
If you're a snapshooter, forget this big honking thing. Your camera's built-in flash is all you need for digital. If you want to bounce, I use the SB-400 myself, and with how well digital cameras work at high ISOs, I rarely use any flash. I use flash only to fill-in shadows in harsh outdoor light.
If you're a serious hobbyist, you still don't really need an on-camera flash like we sometimes did back in the 1970s before film got fast. If you insist on having a big flash on-camera, go for the SB-600 instead. The SB-600 does everything anyone needs, without all the fluff features that get in the way on the SB-800 and SB-900.