I am sure many of us would be upgrading users from KDE 3.x, I am one of them. But the thing is there are still tons of KDE 3.x applications out there not yet ported to KDE 4. It is nice to have KDE 4 running at my book here but nevertheless a few applications does not work/unrecognized at this new barn.
My opinions are, first KDE 4 is fresh out of the owen, pretty much time is required for more applications to be port over.
Secondly, I noticed there are some speed improvement when loading applications, not too sure whether caused by new KDE libraries that shipped with KDE 4 or otherwise.
Lastly, personally there are few default settings and application KDE 4 should take into consideration for any installation because as just now after login into the window manager desktop, I have no clue how to turn down the login tone that is going to attract some cops visiting me in the midnight!
Cheers!