I have used sb28 on both n80 and f100. works extremely well. I use a plastic diffuser cover as well, and get very good color and b&w shots. Many of these are shot in clubs, on the dance floor, in low light conditions. I'm happy with the results.
I always use TTL/Matrix exposure. Strangely, other settings just don't give the same results.
I do not yet have 28dx or 80dx for my d100. However, the on-camera flash works just fine in TTL/Matrix mode. I have not tried variations, such as shutter timing or spot metering - strictly TTL matrix.