I must admit that I haven't used a mac other than some cursory play in front of one. Since it is basically a BSD variant running Xwindows with Mac's stylish interface, I think it is a pretty good choice.
Personally, I use Linux as my desktop machine simply as I have a portable development machine. I can run all of the applications that I need to do any development, and, Linux itself has been pretty good for me. It is not as polished as Mac OS/X, but, on the same hardware, it appears faster than XP is. There's a ton of software out there for it and with Wine, Crossover Office or VMWare you can retain the ability to run Windows if you need to. You can also set up a dual boot system so you can switch between the two.
Gimp is supposedly very similar to Photoshop. OpenOffice.org is like Microsoft Office (less the 'accessdb' portion -- if you need that, you need to buy StarOffice. OpenOffice & StarOffice also run on Windows & Mac), there is GNUCash which is very much like Quicken, Firefox (which runs on multiple platforms), etc.
Unless you're a bit technical, except for Xandros or one of the Retail/Consumer Linux versions out there, Linux can be a slight hinderance for people that are used to working in a certain way. There is a certain pleasure in knowing that 99% of the viruses out there can't really do much -- although if you are running IE in crossover Office, and IE is compromised, Linux/Crossover certainly won't stop the spyware from running.
