This is another hot topic on the blogs and forums that discuss such things. See the slashdot discussion for examples. The reason people use the Ubuntu flavor of Linux is that it consistently gives you an upgrade every 6 months, so you don't have to be way behind the new, improved versions of programs, but 6 months is usually enough time to get a set of tested applications together.
I installed my laptop last week, and have not managed to solve the problems that my fairly trivial usage on that system shows up, so it will be a while (or maybe never) before I put it on my desktop.
The most obvious problem was that the screensaver was asking me for a password whenever I came back to it. I couldn't find a screen where I could tell it not to do that.
I also found a couple of times where the machine was hung when I came back after being gone for a while, so I removed xscreensaver, and went with gnome-screensaver, which does have a screen to tell it not to ask for a password, but it ignores it.
The machine also hung in gnome-screensaver, so I told it not to run. But it's still asking me for a password.
This is not behavior I can put up with on the desktop, and if just going away from the computer is going to break things this badly, I would say the distribution probably got inadequate testing.
On the other hand, I haven't yet found any major problems when I'm actually using the computer. Firefox, rhythmbox, and Adobe digital editions under Wine all work fine. There's one little glitch with the firefox packaging -- you expect there to be a little firefox icon at the top of the screen. There is a firefox button, but it's blank. This is the kind of problem you expect to encounter in a new distribution. Changing your screensaver options on you with no path to get them back again is not.
Apparently there are people in the world for whom this upgrade was easier than the one last April, but I'm not one of them.