i use kde4 with mandriva linux, at the moment with 3gb ram and 4gb swap space along with a 2.6 single core cpu
i've found it very stable and reliable for all my needs, works as it should with no lagging or slowness,
using the 'kick off' style panel is better (i-m-o) than classic style as its easier to find your applications,
programs are virtually the same for all kde desktops,
all will have k3b, konqueror, amarok, kmail, kwrite, etc,
i have pclinuxos2010 kde4 installed on a laptop with only 512mb ram, does the job well - although a little slow, but not slow enough to be an annoyance
as for pro's and con's - any user can only comment on the version that they use,
i use mandriva and it has never let me down,
pclinuxos had a problem on the lappy with sound but alsaconfig from a terminal fixed it, otherwise no other problems
on the other side - i have never got any ubuntu derivative successfully installed on my system without a lot of command stuff,
restricted and proprietary software is mainly down to the version of linux you are running,
pclinuxos comes with most,
mandriva comes with restricted drivers but for other proprietary software then an extra repository has to be added (the plf repo's)