Without the intention to start a flamewar, I'd like to add my two cents in favor of KDE4 here as I haven't read alot in this direction in this thread

. I have first encountered it on my wifes ubuntu machine and found it horrible (she shared the sentiment and moved over to gnome). Since 4.3 has been stabilized, I gave it another shot and was very pleasantly surprised. After changing some settings, nearly everything which annoyed me at first is gone (e.g. the new abomination of a K-menu), and even with some moderate desktop candy enabled, it feels snappy and actually _faster_ than KDE 3.5. If you don't like the desktop effects, fine, just switch them off, it's even a tad faster then.
I consider myself something like a power user, using the machine all day and working mostly via the keyboard on many desktops with many terminals and applications and have yet to encounter an instability or crash, and the only problem (compositing mode broken after being temporarily deactivated by powerdevil while xscreensaver is running) was easily worked around (disable the automatic suspension of compositing). The handful of KDE 3.5 applications (kprinter, amarok, k3b, kaffeine) which I'm still using continue to work without a flaw, and, as a whole, the desktop feels actually better than KDE 3.5 did for me. So, no complaints here.
I can understand actively that maintaining both branches would be a pain in the a** for the gentoo developers and, even if upstream is still maintaining it (a question which kde.org failed to answer at least for me), I'm pretty sure that support will be dropped in the near future. So, like it or not, deprecating KDE 3.5 looks like the correct move to me. If you don't like it, then there's still gnome, xfce and friends which are viable alternatives - after all, you're not bound to a single desktop environment on linux.