I use GIMP a little, but can't really add much here except that I find it does take a while to start up.
On Windows XP I used it a reasonable amount, have used it a little under Windows 7.
How long have you left it trying to start up? Opening it just now took over a minute on my 2.4Ghz quad core Windows 7 x64 machine.
I've not had it fail to start - yet - although as I said, I've not used it a great deal so far.
Ian