KMail is apparently quite a decent mail client, brought to us by the KDE project. If you are one of those GNOME people, you probably want to use Evolution instead. Anyway, there is nothing wrong with MozillaMail.
KMail indeed is pretty decent by now (KDE 3.1). It's fast, has good filter handling, PGP, addressbook, HTML mail (with caution) - pretty much what a daily news client should supply. Is a nice default mail client for the KDE environment - most users should be happy with this. Mozilla is too bloated for many people.