. . . The higher-end speedlights (SB-600, SB-700, 800, 900) have an AF assist light that works better than the camera's built-in one if you like to use flash.
Yes, I have both an SB-800 and an SC-29, both of which have great, built-in IR illuminators. But, I'm often shooting in very low light, with neither my SB-800 or SC-29 attached, and it's difficult to auto-focus.
I wish there was a separate, Nikon IR illuminator-only product which looked/worked just like an SC-29 (without the cable), that you could attach to your hot-shoe, and became activated when half-depressing your shutter release. I know that Sony (or someone else, not Nikon) made an IR illuminator in the past, but you would have to keep it turned on all the time.
Wait a sec.! Couldn't I use the SU-800's IR illuminator to perform a simple AF-assist function? I mean, I wouldn't be "commanding" any other flash units (nor would I be firing the built-in flash, obviously). Does this work?
P.S. I've been dying to buy one of these (for the actual "commanding" capability for which it was designed) for as long as I can remember, but never got around to ordering one--this is an excellent "excuse" to get one now!