Karmic Koala is one of the best Ubuntu releases so far. Some reports might say otherwise, but my personal expierience says otherwise. It's not perfect, and some things are not fixed yet, at least the development is open en every 6 months a new release comes along improving upon the previous installment.
There are some things that bothered me, luckily fixes are available. First, when using the latest Eclipse some button clicks are not registered. This can be fixed by defining the following environment before starting eclipse;
Second, on 64bit the flash plugin also sometimes doesn't register mouse clicks, this is even more bothersome. In Eclipse at least you could still use the keyboard, not so in flash. To fix this you need to add the above line at the beginning of '/usr/lib/nspluginwrapper/noarch/npviewer'.