A bit of a broad question, but I like mine. It's outstanding through ISO 800 and very good through 1600. Image quality starts to degrade after that, and it's no match for the D3/x/700 by 3200. Great focus tracking ability, which is something I took awhile to figure out but now use a lot. I like the ergonomics of it a lot, and the positions of most (but not all) the controls.
It's a camera worth getting, if you can afford it. Otherwise, the D90 is a good bet.
i use the d300 during the day or formal shoots with flash indoors. I really love putting tele's on it as I get more reach with 1.5x crop. My d700 gets used at night for high iso. I've been shooting with my d700 so much at high iso I look back at my d300's 800iso and think "man that looks like crap"