I shot two weddings this weekend and the SB-900 didn't even make it out of my case for the second one. It overheats unbelievably quickly, after hardly a dozen shots at full power, and then it does a thermal shutdown -- meaning it emits a series of pinball machine-esque beeps and then refuses to operate for a few minutes until the internal temperature goes down far enough. Pretty bad if you're in the middle of taking group photos, and twenty pairs of eyes are on you. What are you supposed to do -- tell everyone to wait around until the flash is good 'n' ready again? With this limitation, it's just not a tool that's remotely acceptable to a pro.
(Of course, if you don't shoot events, and can take your time between flashed photos, what is a critical point to me may well be unimportant to you.)
I suppose it makes sense for Nikon to err on the side of caution, but I'm glad my SB-800s seem to be made of sturdier stuff. True, you can't keep flashing away with the SB-800 either -- Nikon recommends cooling its bigger speedlights for at least ten minutes after bursts of heavy and sustained flash use -- but I haven't run up against a practical limitation. I work my SB-800 speedlights hard, and so far (four years and counting) I haven't burned one out yet.
As for the SB-900, it's surprising that a flash this expensive is such a delicate flower, so prone to overheating. In that regard, it may actually be just a little too big & powerful for its own good.
I believe you can elect (somewhere in the menu) not to have the thing turn itself off when it gets too hot, so perhaps you can wring the same performance and stamina from the SB-900 that you can from the SB-800. But while it'd be no fun to burn out a 300-dollar flash, it'd be especially painful to accidentally bump off its considerably more expensive big brother. So I'm going back to my three SB-800s, and the SB-900 is going back to Amazon for a refund.
Get more detail about Nikon SB-900 AF Speedlight Flash for Nikon Digital SLR Cameras.
