I hate reinstalling, because I have so many apps that I use. I've learn to setup my network as centralized as possible, so at least I dont need to do stuff like restore email, files, etc as lot of my stuff is on my linux server which is mirrored (to a certain extent) to my backup server. But stil, reconfiguring my profile settings, restoring thunderbird, firefox, etc profiles, reconfiguring my FTP client sites.... all that stuff is tedious. I take it one thing at a time as I need them though.
But as I start running into small issues, slugishness, lot of junk installed/uninstalled (those always leave crap behind) I tend to start thinking of doing a fresh install, though I usually try to avoid em. With win2k I've lasted about a year between reinstalls. I recently upgraded to XP so I cant tell how long I'll last with XP but its not as fragile as I expected it to be. I probably just got that thought from seeing so many trashed XP machines, but I have to consider the level of the owners as well... spyware downloaders, virus openers, those type of people. Any system can be trashed easily by people like that.
Reinstalling a server is probably worse then a workstation though. When I migrated to FC5 from debian, its just such a long and painful task to reconfigure everything. Though this is where I really like linux, since importing stuff like user databases is literally a file copy operation.