I haven't used it much, but I think GIMP is getting pretty good. Not sure I would encourage someone to use it professionally yet, but it's certainly good enough for personal use.
The main reason is not the quality or capabilities. Anything you can do in Photoshop, you can probably do in GIMP, but the difference is philosophy of design. GIMP, like Linux, is very "figure it out yourself". If you want something a certain way, do it yourself. Instead of getting extensive tutorials and support from professional photographers using Photoshop, you have to figure out how to do everything on your own with GIMP.
Example: CS5's "content-aware fill" was functional in GIMP for at least 2 years before Adobe released it. It actually can do a comparable job, too, in some cases.
The other difference is the caliber of programmers employed by Adobe. If they spent one week on GIMP, it would be 10 times better, faster, more reliable, etc. GIMP is just not as optimized, and it can crash more often. Another thing you have to remember is that GIMP was written and is maintained by open-source programmers, some of which might be photographers, but most of them are probably photography hobbyists who program for a living. Adobe probably employs many photographers to guide their designs.
Some specifics you might want to check on.. Look at the dcraw project to see how they're doing with the 5D3 CR2 files, since I'm pretty sure the default raw converter in GIMP uses dcraw. Also, research the color management tools on Linux. I know they exist, but I've never experimented with them, so I'm not sure how good they are.
Cinelerra, I have almost zero experience with, so I can't give any opinions there.
