The ease of the transition is directly related to which version level of GIMP you are currently using, how well you know how to use GIMP, and what version of Photoshop you are transitioning to: Photoshop Elements x, Photoshop CSx, or Photoshop Lightroom x.
I didn't like gimp... Used it ONE time then decided I needed some form of photoshop. I had used photoshop YEARS ago but it was a very basic version of it... Gimp was much harder for me than photoshop was.
I use LR3 and GIMP at home.
One time when I was with a friend he suggested I give CS3 a go. He said the interface and functions were almost exactly the same.
I struggled like hell with CS3 - I understand the later versions are better but I'm going to stick with GIMP for a while... especially given the price tag difference (GIMP = free - CS5 = £500+)
I was using PS for a couple of years before I knew about GIMP. The transition to GIMP was a bit quirky. I'm less familiar with PS these days compared to GIMP. Other apps have come along too, more recently (FastStone, Lightroom, Photivo), so I use both GIMP and PS less now.