Doing Graphics is not my main job description, far from it (somewhere between damage control and janitor is more like it), but I still do a lot of front end development, at my job I prefer to do all the graphics production work myself for the apps I'm building and have tried using GIMP a crap ton of times over the years - I've never found it to be a suitable replacement. Maybe its familiarity but I've honestly tried to use gimp in this role and just getting around has always been a complete nightmare compared to Photoshop. It may be able to do the job that Photoshop does, but nowhere near as easily.
Having our Devs at our shop use Photoshop is an expense, but like good chairs and big ass monitors, at the end of the day a few hundred bucks every few years isn't really that much and if its that much easier to use then its an expense we'll eat and happily. This also means from our devs perspective the choice between PS and Gimp is not a financial one, and once you take money out of the picture most folks are going to choose PS. So anyway, no I don't think this is a Photoshop Killer.
I keep meaning to go and have a look at the source to see how easy it would be to reskin, because it could be a nice cross platform starting point for anyone wanting to make a really nice image editor, that doesn't ape photoshop, but goes for the less technical users while still having a mass of good functionality. Y'know, for the kids ;)
But otherwise I think it really depends on the features you use. I use Photoshop's layer effects all the time. Anytime I want to point something out on a map or a screenshot I drop it in photoshop, draw an arrow, add some text. Then set a layer effect that makes the arrow and text a bright color with a black outline and a drop shadow. If I need to add more arrows or more text they get automatically styled the same.
AFAIK that's not simple in gIMP or even available.
Similarly I use photoshop's non-destructive layer adjustments all the time. I don't know how I'd get by without them.
My understanding is gIMP will be adding those some day. 3.0 maybe?
What I've found to be the case in prior versions is that it's not sufficient for many minor edits, because it's not 100.0% compatible with the PSD file formats. Even 99% isn't enough in some cases, unfortunately. Since I'm typically editing a file created by a designer in Photoshop, and since I might need the designer to build on my changes, I don't trust anything less than 100.0% compatibility.