That's a Windows thing. I had a Mac for three years, upgraded in place from 10.4 to 10.5 to 10.6, and it got faster over the years. I finally had to reinstall because of a dead hard drive, took me only a few hours to have the computer exactly back to where it was with all accounts, and most of that time was Time Machine restoring a couple hundred GB of data from the backup. If I'd told it to, it also would have restored all of my applications.
So, the time question. Over the course of a few years you can be reinstalling Windows a few times, and each time taking maybe a day to set everything back, including reinstalling all your programs. Or you can just get a Mac, never worry about the annual reinstall, and when you do have to, it's push a button, walk away, and everything is back to where it was before.
Ya know, I had always figured that comparison was a little too extreme. But a couple months ago I finally decided to upgrade my Macbook from Leopard to Snow Leopard, and put in a larger hard disk. So I put the old hard disk in an external FW enclosure, put the new blank HD in the laptop. Loaded 10.6, and when it prompted for "Do you want to migrate from another Mac?" I pointed it at the external drive.
It not only moved all my personal (user) data, but it migrated all the applications, licenses, passwords, keys, certs, etc. The only app I needed to re-install was Thunderbird, because I was upgrading it at the same time.
I was very impressed by the success of the Mac's migration, since Windows applications cannot be done that way -- it's always a re-install because of the myriad customized Windows Registry settings, etc. Apple's application installation scheme is much simpler in some ways, and lends itself to easier install/uninstall/migration. I really didn't think it would be that easy; almost shockingly so.