^^ Software really isn't an issue anymore with the Intel Processor based Macs. Parallels for instant access and if you need more juice you can boot to Windows with Bootcamp or VMWare.
The only big drawback IMO to Macs are the price, but as the saying goes, you get what you pay for. Oh, and maybe games if you are one of those gamers trying to squeeze ever bit of FPS out of your box.
OSX is much better visually than XP. I don't like Vista. The OS prompts you way too much for EVERYTHING. The GUI is much improved but it is a major resource hog. Games are currently showing worse performance benchmarks on Vista as opposed to XP (FWIW, I have 4 GB of memory on my PC box). I won't touch Vista until a Service pack, or two.
