I've had my D300 since December '07 and have taken thousands and thousands of photos. I needed a camera that would be relatively impervious to dust, dirt, rain, vibration, bumps here and there, etc. since I take in on all my off-road forays. It hasn't disappointed me yet! Embarrassingly I accidently (sure wouldn't do that on purpose now would I!) dropped it...and it proved to be 'drop test' tough! I can't say enough about the quality and toughness of the weather seals and overall construction!
Having said all that I would upgrade if it had HD video. I just want the ability, without having another device hung around my neck or waist, to take short vids now and then. I understand through the rumor mill the D300S or D300X (whatever they will call it) will be introduced this coming September: Hopefully it will feature HD video.