I am a GIMP user and that is all I know. I use it for scrapping and photo editing. It's true that it is NOT user friendly and you will have to dig around the web for tutorials. I have adapted photoshop tutorials for GIMP and got along fine that way. However, if you absolutely CANNOT live without styles/actions (for PS) and presets (for LR), then you may not be happy with GIMP. GIMP runs fast for me but then again, I am a NON-windows users. I have used GIMP on a Windows computer and it does run slightly slower. It's probably the same as Photoshop. GIMP does not have some of the advanced features like PS - e.g. content aware, some of the selection features, actions/styles, and probably more that I am not aware of (I have not used PS or PSE at all). But it works for me.
From a designer standpoint, hands down, you need photoshop. It's much easier with the advanced features to get what you need done for designing. GIMP would be just too many steps and you may need some of those advanced features that only PS can offer.
For RAW editing, there is a program called Darktable which is very similar to Lightroom. I have it and used it. I like it although I don't do as much RAW editing anymore. There's another program that converts the RAW so that you can use it in GIMP (similar to ACR for PS) - it's called UFRaw. I've used that and like it.
I read through the comparision that Jenn posted (GIMP vs PS vs pixelmator) and from my humble opinion, this is more for the people that would be creating (such as our designers). For scrapping and photo editing, you would not need to be creating logos. Also, GIMP is not set up to deal with vector images. I would recommend Inkscape for that (another alternative for Illustrator).
Check my gallery out if you want to see layouts in action using GIMP. All of my layouts are 100% scrapped using GIMP. I can use psd templates as well. However, GIMP does not recognize layered TIFF files so you will have to keep psd files when you use templates.
All of these alternative software that I've mentioned will likely have few tutorials or support although it does exist - just not to the extent that PS/LR/Illustrator have online. But if you're willing to learn and get through the trial and error - its a good alternative. The best part is that its available for free. However, it doesn't hurt to donate to the creators of these software - without them, none of it would exist
