Well, I've been running for almost a week the KDE SC 4.5 RC1 and I must say that the "feelings" are *fantastic*. It's almost the same sensations I had when I tried the 3.5 release some time ago: there is both a performance and usuability great improvement on this release.
The most significant improvement I've found is KWIN related: it is finally usable! Not only that but it's actually FAST (both effects on and off) even on my poor Intel 945GM!!!
Another great thing is the ability to use webkit as and HTML renderer under konqueror. It is still not stable on my machine(no webkit based browser is, so it does not seem like a KDE's fault).
What else? Mmmm the networkmanager applet has been changed and cooperates with knetworkmanager.
There are also changes on the Desktop Settings arrangements as well as some minor GUI/shortcuts changes on different places (like konsole).
Well, here are the screenshots: