They have very limited upgradability, true, due to the form factor. Have you seen the latest iMac? Its an incredible piece of engineering. I got one for my gf for Christmas this year. It runs completely silently, is fast and has an absolutely gorgeous screen. Sure, there are only 2 RAM SIMM slots (ok, so that limits you to what, 2GB memory?), and only one drive bay... you can pull the stock drive out and replace with a 250GB SATA for $130 these days. Or, go external firewire. No floppy drive.. Mac's haven't had them for years. The only negative I can think of is that the display graphics probably aren't up to snuff with the latest 3D games.
And let's just add... no viruses, trojan horses or popups. That alone is worth the price premium to me.
http://www.apple.com/imac