Before everyone starts to spread their opinion about KDE 4.0, let me spread some reminders:
KDE 4.0 is not KDE4 but only the first (4.0.0 even non-bugfix) release in a years-long KDE 4 series to come.
KDE 4.0 is known to have missing parts or temporary implementations (eg. printing, PIM, Plasma).
Most changes happened under the surface and cannot be discovered in a "30 minutes usage" review anyway.
User interfaces being unchanged in 4.0 compared to 3.5 may be still changed/improved during KDE 4 life time.
KDE 4.0 will not be the fastest KDE 4 release - like for KDE 2 most speed optimizations will happen later during KDE 4.
Most applications (many are not even fully ported yet) will take only advantage of new features which the new Qt/KDE libraries offer later.
Don't measure portability success (eg. MS Windows) by current availability of application releases, they will come.
KDE 4.0 is only expected to be used by early adopters, not every KDE 3.5 user (and IMHO KDE 4.0 shouldn't be pushed onto other user types like planned for Kubuntu ShipIt (which by the way is said to have only 6 months support for its packages)).
KDE 4.1 development will not require the same amount of time as the big technology jump of KDE 4.0: expect KDE 4.1 later this year.
Last, again: KDE 4.0 is not KDE 4.