For instance the fact it requires a default reply-to address, which makes it nigh useless when working with multiple accounts... It seems old-fashioned and clunky to me, and I would certainly choose Thunderbird over it, and possibly even OE.
I see ThommyTulp has succinctly answered this in the interim - thanks, TT - while I went off and asked the same question in the Yahoo! Groups Pegasus area (the real support is by mailing list, but that's too busy). Here's part of the reply. It's message 43, but you have to join the list to see it, so no point in a link:
=== begin quote ===
> You do not have to enter a default repl-to address. You may do so
> if you want to, but you are not forced to. If you have not
> specified a default reply-to address, the replies to your
> messages are sent to the From-address of your message (i.e., to
> your e-mail address). Setting up a default reply-to address is
> useful only if the reply-to address is different from your actual
> e-mail address. An example: in a mailing list, you want to make
=== end quote ===
It seems old-fashioned and clunky to me,
Well, it -is- one of the older ones. Who wants to be a fashion victim anyway...