Thanks for the comments John! I have used The Gimp substantially more extensively than Photoshop, so I'm probably not all that qualified to make an accurate comparison between the two.
My general feeling though, is that Photoshop will probably always be more fully featured and comprehensive. That said though, the strength of The Gimp is, of course the price tag and that is one of the main reasons I use it. I actually run it under Linux, after doing the basic processing in RSE first under Windows. I have tried the Windows version of The Gimp but have found it to lack stability (it has a tendency to crash, at least for me), but the Linux version seems very solid and dependable.
It will be interesting to see how long The Gimp lasts me, as I learn more and more about post-processing. I may find I get to a point where I feel really limited by it, and end up forking out the pounds for photoshop. But for the time being, it seems to be doing the job quite well for me.
If you already have Photoshop, I'm not sure there would be any advantage in going to The Gimp, particularly if your day to day OS is Windows.