Well I've been playing around with Nikon's latest Speedlite for the past few days, and it's definately got some great features and updates over it's predecessor the SB-800. I have two SB-800's that I've used fairly extensively on and off the camera, and originally thought there would be no reason for me to upgrade to the new flashes as I usually set my flashes manually and don't use the CLS system (more on that a little further down) as I prefer the capabilities of my Pocketwizards (more on that later too).
Some of the obvious changes with the new model is the size and the removal of the side carriage for a 5th AA battery as well as a new interface for controlling the flash on the back. The SB-800 had the 5th battery carriage as a way to keep the flash refreshing faster (meaning allowing you to use the flash again more quickly) and it really was necessary for anyone who needed to take more than 1 picture every 1-2 seconds (which is just about everyone, except maybe...no everyone needs that). The new SB-900 has completely new electronics inside which allowed Nikon to eliminate the 5th battery and still increase refresh time between flashes. This is great for me as I hated having to have so many extra batterys with me all the time and have three leftovers from a pack of eight instead of being able to use a pack of eight to power two whole flashes. The Nikon flashes also have always been a step above the Canon flash system in being able to quickly change settings and setting up a wireless link between the flashes for off-camera lighting. This new model puts a quantum leap between the two, usually very comparable brands. (Also: the SB-900 is still compatible with external battery packs, although I don't use them, and therefore can't comment if there have been any significant changes to the way this works...looks like the same port though)
The wireless system from Nikon is called CLS for: Creative Lighting System. The two primary reasons why off-camera lighting is important: 1) On camera flashes can cause red-eye in photos and 2) On-camera lighting usually looks very bland, and creates very flat shadows with no dimension to your images. The way CLS works is by having a "Master Unit", which can be a SB-800 or SB-900 or even a DSLR's built-in flash in the D80,D200, D300, or D700 and using several "slave units" which can be a SB-600,SB-800, or SB-900. The master unit sends out a pulse of light that tells the slave units how powerful they need to flash and when, which can be set manually from the master unit or can be set to TTL (through-the-lens) which means that the camera will automatically adjust each flash to what the camera thinks will give it a good exposure.
CLS is a great system for using off-camera flash, however has some severe limitations, such as requiring line of sight and having a somewhat limited range due to it being a infared system. Because of this limitation, I've always used a device called a Pocketwizard to remotely fire my flashes as this device is a RF (radio-frequency) triggering system than doesn't have the limitations of CLS. The downside of Pocketwizards however is that each flash must be set manually on the flash itself, so if after looking at my photo I decided I want a particular flash to be a bit more powerful and light up a little more of what I have it pointing at, I need to walk over to that flash and adjust it. I don't have to tell you that this could be a non-optional thing when shooting an important event that is underway, you can't just say "Hold on everybody! I need to adjust my flash!" While this would be fine in a studio type environment or a commercial shoot, a wedding or corporate event would not be the time to experiment and be running back and forth. The product that is out there as the 'best of both worlds' would be a product called a RadioPopper, although I have not actually tested one of those out personally.
I've always found the SB-800 interface to be great, really fast to change settings, and very intuitive (except when making the flash a master or slave...you have to hold down a button for a couple seconds...BUT WAIT! There's an instruction chart built right into the bounce card! AWESOME!). The crazy thing is that Nikon, when designing the SB-900 completely changed the entire layout and design of their old model and came up with a whole new way of accessing and changing those often needed settings. The cool part is that what they changed made a great system even better. Instead of having to hold down the center button for 2-3 seconds, all you have to do now if flip the power switch to the "master" or "slave" setting, and presto...you're on your way. Also they now have a much quicker "Canon-esque (see the SX-100/SX110 series)" click-wheel for going between power settings and adjustments on the fly as well as having individual buttons for each different component of the changes you need to make. One button dedicated for power adjustments, another button to adjust modes, another button to adjust wht your intended aperture will be and another button for the zoom on your flash.
For those of you who haven't used a Speedlite extensively, flashes can zoom just (well sorta) like a lens can. The new SB-900 has a range from 24mm-200mm, which means you can have a very wide spread on your beam to illuminate a whole group of people or create a great fill-light, or you can have a very narrow beam that's great for isolating a small portion of your image with a little splash of light to make that person or object jump right out of the picture. This increased range, when used with lighting modifiers such as snoots, umbrellas, softboxes and gridspots will allow you you keep light as loose and soft or as tight and hard as you want, which just opens up creative control all the more.
Also improved on the SB-900 is the filter holder. Anyone who has used a SB-800, knows how big of a pain it is to fit the included gels into the little hole to hold the gel in place when running around with the flash. The new gel holder actually is a separate piece which goes between the plastic over the flash tube and the diffusion dome, which allows it to be held in place much more securely and not hamper ease of use. I usually set most of my gear manually, so I have no hands-on experience with this, but according to what I've read, the code printed on the gel itself will tell your camera what to change your white balance to, however the second you want to get creative with color, I could imagine that become a big pain to go through and change everytime you change a gel.
Overall the SB-900 is a massive evolution of the SB-800 that is improved in every way except portability, as the SB-900 is quite a bit larger than its predecessor. The only problem I have had with it at all in the few weeks I've been using it, is during a wedding, it stopped firing as it became overheated because we were firing about 15 full-power shots in a row with virtually no wait between shots.
This caused a rapid build up of heat from the flash tube and the flash actually started beeping at me and wouldn't fire for a few minutes until it cooled down a bit. I discovered later I can turn off this built-in safety although it will still beep at you, it will allow you to continue burning the bulb out until you destroy your flash and need to drop another $500 for a new one...but I'd recommend listening when it starts complaining to you about the heat...
Get out there and start flashing! Photography wise I mean...