Seriously speaking though, ubuntu/fedora are usable for a desktop. Maybe not for your grandma, but they're not that bad.
The problems you'll run into are:
- certain specialized software packages are only available for windows, and won't work under Wine correctly (eg, Pro Tools), and there's really no open source alternative for many of those tools.
- less-than-intuitive installation process for installing some specialized software that is available (bring up console, figure out where the OS mounted the CD you stuffed in, sudo bash install-this-shit.sh, add shortcuts manually) since not every software vendor wants to package their product into .deb, .rpm, etc
- drivers for printers/scanners/whatnot (ironically, I boot my home PC into fedora so I can use my scanner, which doesn't have win7 x64 drivers)
- lack of "ask your relatives" help with the OS, unlike windows - you need to browse forums for help with things sometimes, and invariably the forum posts with the info you need have 5 different things get suggested followed by a huge argument.
- Free shit being different than paid shit. Sometimes the subtle differences between MS Office and Open/LibreOffice, or Photoshop/Gimp, can piss you off.
Usable? Sure. However, its somewhat less than Windows at almost every turn.