You can say I love those buttons on the Nikon D90 (which provides better tactile feel to those on the D700) and I push them harder than most around me with a Nikon D90. But there are just that few more buttons I recently discovered I seldom use.
Namely - AE-L/AF-L button next to the viewfinder (and MB-D80) and DOF button below the Fn Button (next to the camera grip). I mean I just did not find enough justification to use these buttons as often as I want. As far as the AE-L/AF-L button goes, I have currently set it to perform a FV (Flash Value) lock role for my SB-400. It does indeed maintain that flash intensity on my subject with each re-composure of the scene. But AE-L? I am not too sure where it's best used...
OK. Say you are at a restaurant and you would like the waiter to help take a shot. You pass him the camera, he aims carelessly on a chair located before your main subjects. He took the shot, only to be discovered by you that it was a bad shot. Sounds familiar to you? In the days of using my Contax T3 (it still shoot crisp, perfect images today), there is a wonderful feature in it to shoot at fixed focal distance (2m, 5m etc). However you compose your shot, all your subjects at that prefixed range turned out razor sharp and well exposed.
I think these AE-L/AF-L button is supposed to do just that, just in a slightly fancier terminology. So, I personally think that the AE-L/AF-L feature is useful only to me in the above situation. The FV lock feature is much more useful to me, programmed on this button.