It is true that I've tested very little KDE4, so I'm giving general impression. First of all, I didn't like the new menu. I don't know why, but it doesn't show you all the apps you have installed, just a few. I don't like, either, the fact that having icons on the desktop means have them as widgets, I think it is not very user-friendly, not seeing anything in your desktop, and also, a place when you can easily have acess to. Now I have to open a folder to take certain file, for example. The problem with the icon widgets, is that you cannot organize them instead of by hand, and they look pretty ugly the way they are. Also the taskbar isn't the best point of it. In the end, I can't understand why they've launch KDE4 if they haven't completed Oxygen icon theme yet. There're a bunch of apps without Oxygen icon and they're using wether a general one or KDE3 version.
Now, the good points. I really like Dolphin as well, I think it makes you do many things and lets you work the way you prefer. In other words: has such a versality, that you can do the layout you want, to work the way you wish. Another thing is (but not of KDE4 in particular) the speed of caching image miniatures. While in Gnome it lasts years to make image miniatures, KDE/Qt makes them in a blink. Awesome. I like also (thinking in the end user) the integration of the compositing manager, because it's much easier for the end user to have it running and configured. Because now we have a startup script and, the CSSM, it's a nightmare of options and names (the exposé-like plugin is named 'scale'). It is just a nonesense, and against any rational usability.