I used the betas, KDE 1-3, skipped 4 an 5, and I'm now back on the 6 train. It's really nice, and I try to use something else every now and then and I'm back in KDE a few weeks after.
5 was a bit rough at the start, but really was pretty nice by the time 6 came around.
6 has been great. If you are a windows or mac user, it's very close to that experience and very familiar.
I don't remember the earliest version I used, but I vividly remember kde 3.5 and 4 because the transition was really rough. Major regressions and highly disruptive at first, but it laid the groundwork for modern kde and haven't had anything like that since. I jumped ship to gnome out of spite but was back to kde in short order.