I've just upgraded to from the Intrepid Ibex version of Ubuntu Linux to Jaunty Jackalope, the latest brand new version.
So far, most of the significant problems have been with sound. It initially wanted to send all sound to my USB phone rather than to the speakers. Not sure why it didn't keep my old settings, since this is an upgrade rather than a fresh install, and it was still using Alsa. I've decided to give PulseAudio another chance--Last time I used Pulse, it would frequently go bad, and require me to kill and restart it once or twice a day. Once I got it sorted where to send sounds, it seems to work OK. Audacity music editor now works with Pulseaudio.
Amarok looks promising for a music player if I understand the weighting system right, but it is more complicated than XMMS. It doesn't come with MP3 support standard, a common problem with Linux apps. That is excusable because of the legal issues involved. The gripe I have is that it also doesn't tell you about the problem, it just skips all the MP3's without playing them. A quick Google told what to install to make it work. Audacious (the successor to XMMS that doesn't work as well) doesn't work out of the box, where it did before.
Another setting that didn't carry over properly was the LCD subpixel order--it was set to vertical, and that triggered a bug in some apps (Skype and the HP printer applet) that made their fonts completely unreadable. Setting the order properly both fixed Skype and made fonts better.
I now have control of typing repeat rate that I lost in Intrepid.
It appears that the one-pixel line glitches I had with dual monitors are gone (I'd get a one pixel horizontal line on one monitor that was related to what was shown on the other) but they weren't consistently present before, so I can't be completely certain.
Panels (Toolbars) are on my laptop monitor where I want them. Intrepid kept moving them to the external monitor whenever I would disconnect then reconnect.
The install wanted to delete a bunch of stuff that it thought were no longer needed, but gave the option to cancel, which I did. Several of the things it wanted to remove weren't obsolete, but rather things I had installed without using the official repositories. The same options to remove were available in computer janitor, but with decent explanations and individual control. This let me get rid of stuff that was really obsolete, but let me keep XMMS, Skype and codecs.
Flash appears to be much more stable, where in Intrepid I was getting sound problems that required restarting Flash periodically.
So far, not much in the way of amazing features, but solid bug fixes. Much more pleased than the upgrade to Intrepid Ibex.