Being someone who knows a few things, I decided that in order to be fair: I would generally hold off deciding whether KDE 4 is an improvement over KDE 3, until after KDE 4.2 was released. Well, as life has it, I've spent most of two days compiling KDE 4.2.2, and things went very smoothly (not that I liked compiling ocaml among the dependencies :-/). This is on my core workstation, running FreeBSD RELENG_7 (i.e. 7-STABLE).
My very first impression was... is this thing working??? All that first time setup takes a while to do, and without much sign of anything happening in the background. On the second boot up, after logging in through the X Display Manager (XDM), I counted 17 seconds until there was a usable desktop; but user interaction was clean all the way, no doubt about that this system was coming online. If you count the time it takes for korganizer and the other system tray icons to load, about ~45 seconds to get a full desktop -- but it's not far to count background programs like systray icons lol (especially the kind you'll likely remove later).
Although I think the startup time shouldn't be to bad for most people, I've forgotten how long it took to get a full KDE session going up, but I would say 17 sec is pretty good on my hardware hehe. With just a Sempron 3300+, even Blackbox and FVWM2 could start faster for my tastes, so no problem.
I find the new style K-Menu quite useful, takes a little getting used to - learning what adapts a focus follows mouse approach (the nav-icons on the bottom) and what requires a clickly click to work (most everything else). It is beyond me why it defaults to that behaviour (developer preference maybe?), but easy enough to make it a bit more consistent: right clicking the big K and going into the application launcher settings put the desired option right under my noise :-).
Obviously, the first thing I looked for was Konsole, the theme stuff on it is just awesome. Second thing was to dig up the run command dialog to get my urxvt+screen going. Further attempts to use the run dialog, proved that it was mostly a piece of krap. (Eye candy, but shitty to use; guess that is why there are terminal emulators.)
Closing the desktop folder viewer widget-thing was the third major action. Because I'm a person that hates having a desktop cluttered with icons (I prefer terminals :-), I like the idea very much, but since I have no immediate use for it, no need to have it taking screen realestate.
One thing that irked me, bringing up the help and control center entries on the applicaiton launcher (K-Menu?) loaded the ones from KDE 3.5.10, joy 8=). Oh wellk, it's quite easy to remove or change them via the menu editor. In the case of khelpcenter, it seems it just finds the wrong documentation ^_^. Killing off the old thing and setting /usr/local/kde4/bin in PATH at the top of my ~/.xsession file, fixed access to the Control Panel. I must admit, I rather dislike KDE3 cruft in the menu - however you slice it
For years, I have wondered why some systems never turned on the NUMPAD by default, considering that I now do so much off a laptop; I can understand why, it's a pain in the ass if when its unexpectedly on xD.
I generally feel that the whole Plasma and widget crazed stuff is a good element of KDE 4, but in all honestly, FVWM and Blackbox have just spoiled me something terrible.
It would appear, that KDE 4.2.2 is more or less ready for general usage, and unlike 4.0RC*, can actually be customized quite a lot to taste :-). For a little while, I was worried that Gnome might take the lead, and keep it... but I think by the time Gnome 3 hits, KDE 4 will be queen and king of the desktop environments, hehe. For those who desire eye candy, and have a machine capable of it; if you liked AERO, you ain't seen nothing yet laddy. I think anyone who is still holding onto KDE 3 at home, should start migrating while the getting is good; and employ programming talent for bringing along any missing "Must haves" to the new desktop. I am not sure if it is really an improvement beyond the concepts, but hey, at least there is Okular!
As for me personally, well I've gone back to the Famous Virtual Window Manager version 2.5.27, old habits die hard ;-)
Further testing of KDE 4.2.2 and later, will probably be through the Windows builds, rather then assaulting my poor stable laptop hehe.