Well, as far as Photoshop vs CaptureNX, I used to be a very pro NX user, but as soon as I saw the difference in quality that I was getting with CS3 over NX, I can tell you that the main reason is that I personally was getting better results with CS3.
As far as CS3 vs Gimp, I tried looking into it before I purchased CS3 and either I was lost or I found it was more for a Linux than Windows platform (could not find the windows installer, links were all down).
As mentioned, it also sure does a LOT more than any other program out there. If you learn how to use it, there is not much you cannot do.
Besides... CS3 cost me $195Cdn ($5 for a CS2 CD found at a garage sale and $199US for the online upgrade). Thats about as cheap as it gets!