I just recently started to use KDE4 a bit as debian finally moved kde4 into the testing tree.
But my first impression is a bit confused. I miss a lot of the usual KDE3 usage applets and usability in the panels.
But the most i miss are some basic usabilty things ksysguard. I used it in my panel in kde3 as a monitoring device, well seems there is nothing like this anymore (why?) and the second big thing I miss is that there seems to be no way possible to stack bars together. How could the be gone? How else do you want to proper monitor detailed CPU usage (idle, user, sys) or memeory (apps, cache, buffer, kernel)? I am really confused why those core things are still missing in kde 4.2.2
I really hope they will come back in some later version.