I sold my D90 as soon as I saw the D300s was on the cards (and then Nikon launched the mighty D3s, but that's another story).
I have a D300 and absolutely love that camera to the extent that I use it over my pro spec D2Xs. The screen, and most importantly, the files are superior in every way. So to own a virtually identical body with HD video capabilities was a no brainer.
And last week I got to give it a little run out at a local cyclocross event. The results are OK IMO, but could be way better.
I struggled viewing the rear screen in the bright sunshine, which made any tricky focusing with a shallow depth of field impossible.
There is this little item which would make life easier but at over 300 quid I can't see myself buying it anytime soon. In fact the cost of some of the latest VSLR rigs is getting on for twice the price of the camera body itself! Instead, I have just purchased one of these
which is definitely more useful for achieving decent shots from a caffeine addict. I already own a Hoodman Loupe which I reckon I can cannibalise to meet my needs. After that it's look out Scorcese. Or something.