Hi Radar, it was just last week that I bought an SB800 at B&H. My main reason for choosing it over the SB600 is basically it has more auto
modes. Besides TTL support (iTTL and D-TTL respectively for cameras that support either), it has auto aperture, non-TTL auto
mode, guide number distance priority, repeating flash and full manual
modes. The SB600 only supports TTL and manual (it only has repeating flash
mode when it's a slave, and the master fires repeating. NB: repeating flash is distinct from the "modelling flash" which flickers much like light under fluorescent tubes, annoying and not really functionally useful IMO).
I expect the lack of other auto
modes will not be an issue for most people, as they don't have 20 year old manual film SLRs they wish to attach the flash to, at the same time are too lazy to calculate the flash power required manually from a chart... but I'm not most people.
The 2/3 of a stop difference in flash power is insignificant IMHO. Look beyond the published Guide Numbers, and you'll notice both have a maximum range of 20 metres (from manuals d/l off Nikon Singapore site).
The accessories that comes with the SB800 have varying useability depending on your shooting. The 2 thin gel filters (it's not gel, it's more like cellophane thin!) matches the flash output to tungsten and fluoro lights to ease camera white balance may be useful if you do alot of indoor flash work. The main reason I bought a speedlight was for daytime fill-flash (take advantange of that 1/500th flash sync). Daytime flash work requires alot more power (competing with the light from the sun). I still have the built-in on the D70 - which can be used for nighttime when flash power isn't so demanding.
The optional 5th cell with the SB800 is an anomaly. I have a charger (Maha C204F) that charges batteries in pairs. I don't know of a charger that charges independently 5 cells. From an electronics point of view, it's highly desireable for the 5 cells in the flash (if using 5 cells), to be discharged/recharged at the same time. Also, I know of no place that sells
AA batteries singularly! Most come in 2's or 4's. So I'm using just 4 NiMH cells. The 6 second recycle time quoted in the specs is highly conservative. I counted closer to 4 seconds for a full power discharge with NiMH cells. I expect the SB600 to be the same or slightly faster (less powerful maximum).
The soft case that comes with the flash is surprisingly well made. It's not a cheaply finished afterthought, it feels like good quality waterproof nylon material.
Oh, I forgot the diffuser that comes with the 800 and not the 600. While it's desireable not to have that harsh direct flash look on faces, there are ways around it - eg. bounce and the problem goes away. If you insist on firing the flash direct, you could always pull out the wide angle adapter that will soften the light somewhat and duplicate the diffuser. Also, the included white bounce card can be imitated on the 600 - velcro, tape or rubberband a white piece of cardboard or thick paper over the top of the flash and voila! (not elegant solutions, but workable nonetheless). Stofen sells an omnibounce that'll fit the 600 anyhow...
I gtg, if you have any q's, I'll happily try to provide the a's.