Fellow KMail user here.
For my email, I find that KMail is the least annoying mail client. The
font rendering is good, and the layout and icons are decent. It doesn't
seem to butcher diffs the way other mail clients do.
Two things drive me crazy about KMail. The first is that it does, on rare
occasions, crash. It's not nearly as bad as Konqueror, which I also use
because I find it the least annoying... but it does still crash. :(
The other issue is a behavior I've seen when KMail starts juggling a lot
of messages over IMAP. Eventually, it seems that the cache gets corrupted
such that the message header list won't match with the actual message
list. In other words, you end up with surprising behavior in which you go
to click a header (or key to it) and find that the message KMail has
loaded is completely different. Generally, the header you click will
change to match this message as well. The confusion gets even worse when
you try to use the Search feature to search the message headers...
In these instances, I've found that nuking all of the IMAP cache with
rm -rf is the only way to fly.