I noticed that most of the people who recommend PS have done so either by not having ever used the GIMP or by having installed it and using it for just a few minutes. The problem with that is years ago when WP was the defacto-standard for Word Processors (5.0/5.1 for DOS timeframe) I was given a copy to look at, installed it and then uninstalled it, why? 5.0 didn't have a menu and I didn't know how to use it. Once my friend showed me how to use the program I fell in love with it, it was the best thing, and I still long for the days of using such a great piece of software.
GIMP doesn't have all the features of PS, but to say it's not as powerful is a generalization. Alot of that depends on what you're trying to do, what you want to accomplish. The only thing that PS does much, much better than GIMP is 4-color seperation mode, which GIMP lacks (though it can export to CYMK).
GIMP does operate a bit different form PS, mostly becuase it's not native Windows and doesn't use the MDI that most Windows programs use.
With a bit of learning you can apply most techinques that are used on PS to GIMP, and you would be surprised at how many plugins are availabe for GIMP (though many are pre-compiled for Linux and not Windows).
Considering that GIMP is free, that there are several books out on how to use it (check your local library or Amazon.com), and that most people are in this position:
http://archive.gamespy.com/comics/dorktower/archive.asp?nextform=viewcomic&id=923
I'd say that it would be worth most poeple's time to get to know GIMP, unless you can afford PS. I'll have to see if I can find some links for artwork created with GIMP, there was one woman who used to post to the GIMP artwork Yahoo Group when I used to be joined up there who made some awesome paintings, all done in GIMP.