Developers are the wrong people to ask

Of course, it depends on your definitions, but I think you'll be a bit surprised by the
improvements already available in KDE 4.1. I've used KDE 4.1 beta 2 for a few days and except
for stability problems in various places, I'll rate it as fully usable. I tried 4.0 and dumped
it very quickly since I found it to be more or less completely unusable. I'm running Kubuntu
and their next release will be based on KDE 4.1.x and based on what I've seen from the beta
release, I'm not really worried.

Plasma is a highly visible component of KDE 4 and sadly it was also the most unfinished part
in KDE 4.0. It being massively improved in KDE 4.1 also makes KDE 4.1 a lot more usable. 4.1
is clearly still an early release of KDE 4 and you are correct that following releases will be
much better. Especially since by then, a lot of the major KDE applications will have been
ported to KDE 4.