Actually, The GIMP (and the GIMPshop modification) aren't "weak" at all. They're not professional level editing tools equal to Photoshop, to be sure. But depending on what you are wanting to do, more often than not, you don't need the professional level tool, when the alternative works just fine ... and is free.
Having used both The GIMP and Photoshop extensively, I'm comfortable in saying The GIMP is on par with Photoshop 5.5 - 6 (CS2 is Photoshop 9), with the exception of pre-press tools, and CMYK support. The GIMP supports Photoshop plug-ins... so comparing plug-in filters is pointless. The GIMP's scripting/automation is leaps and bounds more powerful than Photoshop's batch routines... but the added power requires you to learn a little about the scripting language.
Basically, unless you have a need for the pre-press and CMYK support of Photoshop, The GIMP (or GIMPshop) will be perfect. If you're just wanting something for photo editing, touch ups, designing graphics for online and/or gaming use, etc, there's really no reason to use the professional tool when the free alternative is also probably more than you'll ever need.
Graphic Design / Photography / Web Design