The price isn't necessarily an issue when you compare it to the next model up, the full-frame D700, which costs about £400 more. But you can get the D90, sporting the same sensor and movie mode as the D300S, for about £550 less, and that includes Nikon's rather good 18-105mm VR kit lens. The D90 isn't as rugged as the D300S, and can't match its frame rate, but it's not going to fall to pieces in your hands and its picture quality is just as good.
The other issue is that this is Nikon's only pro camera with an APS-C sensor. There's got to be one because of all those professionals who've invested heavily in pre-full-frame Nikon gear over the years, but, if you're just starting out in professional photography, the full-frame models are a much better choice. The D300S is bang-up-to-date, but the DX format, at least as far as professionals are concerned, has maybe had its day.
The Nikon D300S is a great choice for professionals who've got numerous Nikkor DX lenses and don't plan on going full-frame any time soon. It could even make a handy second camera for a full-frame system, thanks to its fast frame rate and 1.5x focal factor (it makes your telephotos go further). But it's not cheap, and the DX format is hardly future-proof.
Edited by Charles Kloet