Becuase everyone here has way too much money. I don't know myself why photoshop is the first recommended program when someone asks. GIMP is no where near as powerful and won't handle RAW files, but it is free, and you don't need to sell your camera and all of the camera gear to be able to afford it.
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!
1. I have a mac and to use gimp you need to run X11.(If you run windows or linux you won't have this problem.) Which isn't a real pain but it does have some funny quarks. Like each tool bar is considered it's own window. If you minimize the program and send it to the dock you need to grab all three tool bars to have them open. I know not a big deal but a pain when you have a bunch of windows open and are cycling through things.
2. I've been using photoshop since 1992. Somewhere around that time at least. My version of PS is old so I downloaded gimp to try it out. It's really different. I find it like starting from square one again. I would be working one some photos and instead of flying through and getting things done I was busy reading tutorials and learning the program.
Not a big deal, but I'm lazy and have the means to get photoshop. All this being said. Most of my image editing needs are taken care of by programs like aperture or lightroom. I just scratch the surface of photoshops power. Right now I do most of my editing and arranging in iphoto and resizing in Imagewell ( another fantastic free program) and if I really need to I open up photoshop. Which is pretty rare these days.
Power wise, yeah it doesn't keep up with CS but it *DOES TO* Handle Raw files, well, handles all my Canon raws (.crw & .cr2) fine :-D
I use Gimp a *lot*, its not as powerful as CS but lets remember CS is kinda like the human brain, most people only ever explore 2% of its potential, so use Gimp and you might get close to its limits one day if you try really hard
No doubt CS does some things better but you can buy some nice glass for the price of the newest CS.... I know which I prefer to spend money on :-D
I started learning PP on Gimp because it was free. It is a powerful program and does a lot, but I found that it definitely is not as user-friendly as PS. I did a lot of digging on the net to find Gimp tutorials. There are alot of them out there. Overall, I would recommend it if you don't have much cash.