Here's what a few folks had to say about the new mac-daddy flash, the Nikon SB-900.
Click on any of the links below to read the full review(s).
Downside: price, increase size takes up more room in your bag and your current light modifiers might not fit it (A Scott Kelby blogger also noted that it was a challenge fitting them into Justin Clamps, and some other off-camera hot shoe mounts).
Upside: recycle time of 2 seconds (1 second of you use the optional battery pack), thermal shutdown protection.
"My biggest problem is, I absolutely love the SB-800s. IMO, many of the added features are great. But I do not think I can justify switching everything out wholesale. Buying just one might be a very good idea -- I can see many instances when those extra features would make for a more useful single flash."
Downside: Size is bulky, plastic tripod mount, No multiple cabled TTL or TTL modes.
Upside: Thought it was their best flash interface to date, better swivel, dedicated gel holder.
"After some preliminary use, I don't think there's any doubt: get an SB-900 if you're heavy into flash (and retire your SB-800 to dedicated remote wireless use)."
Downside: Too big, too expensive. does not work in the traditional TTL mode of film cameras.
Upside: Great zoom range, evenness of illumination, Faster recycle time.
"The SB-900 is a flash for full-time newsmen and wedding shooters. It's a poor choice for general photography."
Personally, I have no plans to go out and get the SB-900, as I use flash only when I absolutely have to (like shooting a wedding in a cave), but I am troubled that the SB-900 is a replacement for the SB-800, not in new addition to the Nikon flash family.
You will find the Nikon SB-900 page here.