I use a Nikon D100, with which I am inordinately happy. It's such a flexible system (I previously used a Bronica and 2 Pentax SLRs), and the performance is outstanding. Â I'd be interested to hear what flash set-ups others are using - I haven't splashed out on a Nikon flash yet, and I need to be convinced taht it will make a significant difference to a non-Nikon one.
There are many who think that the Nikon flash metering system is the best around, especially for fill flash in daylight. It accounts for the focus distance in calculating the flash output (IIRC, Canon's current system does not do that, but Minolta Maxxum and post MZ-S model Pentax cameras do), and is excellent at getting balance right when doing daylight fill. Most of the major SLR systems now use a pre-flash to help calculate the output for the actual exposure, and if you use a generic flash you won't be able to take advantage of that. In my experience, preflash TTL (through the lens) metering is more accurate than non-preflash TTL or the non-TTL metering systems, and systems that account for subject distance may provide an advantage as well. If you don't use a Nikon flash, or one of the third party flash units that is built for use with the newest Nikon bodies, you won't be able to take advantage of the flash metering system in the D100. IMHO, that's missing out on one of the best features of Nikon's camera line. Going with a Nikon system flash will at a minimum allow you to take advantage of the metering of your camera, and if you go with the full bells'n'whistles flash models, you can go to wireless off-camera flash which is really cool sometimes (but which very few people ever do or have a need for). And this comes from a person who owns not a single Nikon product. (Pentax SLR gear, Cosina/Voigtlander rangefinder, and Panasonic and Sony digicams).