Apple's Mail went downhill in 10.4, on the whole. The searching is improved, but just about everything else is worse. The improvements to NSTextView to support better HTML editing seem to have introduced a lot of redraw and functional bugs. The IMAP handling seems to have become less robust, and Mail seems to have developed the bad habit of lying about what it's done. Sometimes it will look like it's deleted a message, for example, but when you quit (at which point it hangs for minutes) and restart, the messages are back from the dead. Not even marked read. (I thought other mailers were ugly in putting a line through messages pending deletion, but I'd rather have ugly than untrustworthy.)
I've talked before about my problems with Evolution and Thunderbird.
So I'm pretty receptive at the moment to the idea of a Java mailer that I can run anywhere, and that's likely to be more hackable than a single-threaded C monstrosity. I try to steer clear of other people's unfinished software, but "1.0 release candidate 3" sounded close enough to be worth a try.
Columba's IMAP performance is criminally bad. I read somewhere on their web site that there's no caching, and it shows. Even on a LAN, it was annoyingly slow. This could presumably be fixed. (On the plus side, Columba doesn't default to removing messages from the server, something several other mailers have screwed me with in the past, shortly before being uninstalled as retribution.)
Columba offers the full range of reply/forward options, but I was disappointed to see that the default toolbar has "Reply" (which is almost never the right choice) but no "Reply All". (Apple's Mail lacks the ability to forward or reply without mangling the original message. There isn't even "Forward As Attachment", which is the usual work-around for over-zealous wrapping and the like.)
Columba lets you see text/plain and text/html attachments inline, like Evolution. I like this. Mail only shows images inline.
Columba doesn't use the platform's native HTML editor. Its Swing-based HTML editing sucks. Plain text editing is fine, but nothing special.
Columba highlights the current message in black on dark blue on MS Windows. This, as you can imagine, isn't readable. The current mailbox, by contrast, is highlighted sensibly. (This seems to be true on Linux too, judging by the screenshots available on their web site.)
Columba got itself uninstalled when it corrupted its configuration files and refused to start.
When they hit 2.0 it might be worth another look, but right now, they don't even get a link.