The D90 is amazing. The functions it has are WAY beyond anything I know at the moment and truthfully I need some time and/or a class on figuring it all out. The only bad thing is the auto function isn't perfect, but if you have such a nice camera, its a shame to use only the auto feature. I typically use aperture priority with some custom settings.
The nikon lenses are amazing as well. Their vibration reduction technology is nice, especially for someone like me who has unsteady hands (which I HATE..) I have the one that comes with the kit 18-80 I think and a 70-300mm zoom lens. I love the 70-300, good quality, a bit slow on the F-stop, but for nearly everything is more than sufficient.
I would get it. One bonus about the D90 vs the D300, which is also great, is the weight. D90 weighs a lot less than the D300 and the price point is a bit lower.
Really I have no complaints about the camera, love it, would buy it again.