I've been using Kubuntu Karmic since the beta, and it's been working fine for me. I've had precisely two problems: Slightly flaky sound (fixed by installing PulseAudio and using that as the default over whatever KDE defaults to), and the kernel bitching a bunch at me about having ECC disabled in BIOS (fixed by blacklisting the ECC modules in /etc/modprobe.d/blacklist.conf).

ext4 is fully mature--it's no more "bleeding edge" than the 2.6.28 kernel used in 9.04--but you can choose ext3 if you want, and if you're upgrading it won't in-place upgrade you to ext4. Grub2 was an interesting choice, though if you're updating from an older Ubuntu version, again it won't upgrade that for you. Grub2 works fine for me; it's just a bunch slower than legacy GRUB.

This depends on how well the menus are designed, too. One quirk that's bugged me about Firefox for a long time: In *nix systems, the preferences window is called Preferences and is located in the Edit menu. In Windows, it's called Options and is located in the Tools menu. I know they're trying to emulate the paradigm that other programs for [Windows/GNOME/KDE] will also use, but that d'oh moment, when I'm telling my (Windows-using) grandpa how to change his home page and I tell him to find Preferences under Edit, is pretty annoying.

I think more than a few people would be willing to use BSD, if the driver situation weren't so bad for it. It's worse than the situation Linux is in, and Linux has some severe driver problems as well (just look at ATI video cards).

You forget. We're the descendants of the useless 1/3.

could an AM2+ processor be used in an AM3 motherboard?

Nope. The AM2+s lack the DDR3 memory controller.