I have an SB600 as well. It works with D40, D60, D90, D3000, D3100, D5000, D90, D7000 and everything else I can think of that's been released in the last 5-10 years. Not sure about the film cameras.
The issue with the SB600 is that it doesn't have a bounce card built into it like the SB900 does.
You can pick one up for $200 I think. I bought mine for less than $200 brand new but since I work in a photography store, I get the supplier price from Nikon.
My SB600 actually lasts quite a long time with one set of batteries. I have rechargeable batteries but I usually only need to recharge them every 2-3 shoots.
Nikon has a bunch of new flashes now I believe. I haven't seen them yet though so I have no idea how they perform. From what I heard though, they are faster (recharge) and less prone to overheating than the previous ones.