I already wrote about how Thunderbird's great win over Agent is its IMAP support, the reason I'm looking to move to it. And I've written about the biggest place where it loses, the way Agent handles mail routing to folders so intelligently. What about everything else? Well, it's a really mixed bag.
I'll give Thunderbird credit for doing HTML rendering far better than Agent. That I can compose HTML emails might be useful, though I don't intend to do more of than than I have to. One thing that Thunderbird wins on is kind of embarassing for Agent: with a simple plugin, it minimizes to the system tray, easy and perfect. Agent could only do that with the addition of third party programs like TrayIt, all of which had one flaw or another. It's embarassing because, given the development system they programmed Agent in, it's about ten lines of code to add minimize-to-tray, and I even emailed them the code. There's no reason for them not to have added it.
Thunderbird apparently wants to make it easy to move from another client to Thunderbird, provided the other client is Eudora. (Though from what I see on forum posts, even that only works half the time.) It's trying so hard to be so smart, that it actually is broken, and seriously, "what the hell were they thinking" broken. First, it won't import any standard mail formats like the most basic of all, the raw email dump defined in RFC822, where email was invented. Not even newer standards like XML. Not even widely-used proprietary formats like .msg files. No, it only wants to read four formats, from four specific programs. Second, it won't let you tell it where the mail is you want to import. It insists on trying to figure that out for itself based on the registry keys that some versions of some other email programs have installed -- but not even all versions of those programs. It can't find the files unless you figure out how to trick it into looking where they are. Third, even if you fake up those registry keys, it can't import them most of the time anyway. It's so dependent on some very specific setup of some particular versions of some particular programs that it can't just import the mail and be done with it. It's too smart for its own good.
I eventually figured out a long, byzantine series of steps involving exporting one folder at a time, closing Thunderbird, dropping the export into a specific filename, reopening Thunderbird, then moving the messages on to their final destination, then repeating the whole thing. In the end I've spent hours on doing what should take seconds. If it just would give me an Open File dialog and then import whatever I say, I'd be done by now.
I don't deny that Agent is still mired in the past, and the loss of development makes it a dead end. I don't deny that IMAP is a necessity. But it's a pity that someone couldn't take the brilliant things Agent was doing, instead of doing the same things everyone else did (like IMAP), and steal those ideas. They really are good ideas. And I wish Thunderbird would stop trying to be smarter than me in areas where it isn't and can't be. Sure, have an Import From Eudora wizard, but don't cripple all your importing with the wizard format. Help or get out of the way, Thunderbird.