I'm the typical Windows power user, and I've attempted to use linux several times; most were part of school and training though, but on one occasion, I've installed Ubuntu on one of my laptops.
At first, Ubuntu came close to giving me headaches. I was having issues getting it to enable the wireless NIC. After two nights of googling and trying to remember commands in the command line, I was finally able to find a solution. The solution involved going into root, which I had more issues doing so. I believe I had to boot up in the command line safe mode thingy and set a root password, and then log in as root to be able to type the command that enables the wireless NIC.
As I said, I'm a typical power user, and I've had some formal training on other linux distros (Red Hat, Fedora, and Suse), and it took me that much effort to get my wireless internet working. Based on this, I doubt your average end user would be too comfortable with Ubuntu, and perhaps other distro's as well.
Obviously, because of my experience, I don't really like the getting rid of root thing in Ubuntu. sudo never worked for me for some reason. The concept seems nice, but "the **** just don't work for me!" Another thing I didn't really like too much is the apt-get system. Half the apps I've tryed failed to function at all, and the other half did absolutly nothing I needed them to do because they lacked a good description of what they do. With my little experience with Fedora/Red Hat .rpm's, I would have to say that I like them better than apt-get. I want quality applications, not ****loads of nearly useless, compiled-in-parents-basement applications.
I'm saying linux is not ready for "prime time" because if it gives me such a hard time, I certian it will give my grandmother a heart attack and drive my mother nuts if they were to try to use it. Naturally, I will be trying different distro's of linux in the near future; it can only get better and I can only improve my skill in using it. I believe linux will, for quite some time, remain an OS of power users and enthusiast.