I've not handled a 40D or 20D or whatever - But the D300 is based on the D200 which I had at the Worlds in Japan and it's definitely a nice meaty strong body. Good enough to take out in the rain and not worry if you drop it
. I've dropped my D3 - I managed to break a grand worth of lens in half but the body was fine
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I think the Nikon cameras are nicer built and feel nicer to use than most others- but hey, that's me. The D300 isn't far off the D3 in fact I think it sits alongside it as the top line camera (the D3 is full frame obviously) - so it has the same but more useful focusing (better frame coverage than the D3) and I think if you add the battery grip at a later date you can get 8fps out of it, which puts it along side the best sports cameras around.
If I didn't have the D3, I'd have the D300 for sure - it's ace!