I have been using KDE 3 for a number of years now, and I have been testing KDE 4 on and off for about two years. Over the past two years, I have had test systems containing KDE 4, but about a month ago I replaced KDE 3.5.10 on my main system with KDE 4.2.2 (on sidux, which uses Debian Sid packages).
There were a few issues at first, but the desktop continues to improve, and it is usable enough for me to use on a regular basis. I do have other desktop and window environments available in the now unlikely event that I run across a serious problem, but I've had the system running continually this session for 2 1/2 days, and I've run it for a couple of days in a row before - now to run it with some longevity and make sure it still holds up.
I was also curious to see how well KDE 4 can do on older hardware. I have a Dell Dimension 4100, containing a 1 GHz Pentium 3 processor and 256 MB of RAM. I started it up last night, updated one distribution on it with the latest KDE 3.5.10 software and noticed that while it was not fast and it had some swapping, it was not terrible and it actually worked fine, just not as fast as my most current Duo Core systems with 2 GB of memory. I wanted to see if KDE 4.2.2 would do as well, so I upgraded my sidux instance on that system and started KDE. It still works fine, so a Pentium 3 with 256 MB RAM remains a viable platform for using the current implementation of KDE.