yay more email clients
Sorry. I'm cynical about this new email client thing that Brent has kicked off. Don't want to be a source of stop energy. But quite aside from my normal IT'S DOOMED instincts, I think they're solving the wrong problem.
There are people on the list saying that an email client needs to be aware that people have 3 computers and a phone nowadays. There are people wanting it to be properly mailing-list aware so that you don't have to set up manual filtering rules. There are people wanting it to be more understanding of current email conventions, so it (for instance) will trim automatic mailing list footers from replies so you don't get 30 lines of repeated cruft.
But the first of these goals undermines every other clever feature. Most of the current problems with email are inherent to IMAP, because IMAP is just a heap of folders with too many configuration options (folder prefix, for instance..). IMAP doesn't do magical mailing list filtering, so it doesn't matter if I have a clever client that does, because my phone won't benefit from any of it. And if I reply to a mail from my phone, the footers won't magically get trimmed, my phone doesn't do that. And if your phone doesn't sync with your desktop address book, you won't be able to compose mail to your friends anyway.
Google Mail gets this right. They don't do IMAP except as a backwards-compatible API to their mail store. They got to start again, and properly reinvent what mail is. Mailing list and spam filtering is done on the server side, rather than relying on the client to pull down all your mail, sort it, and push it into new folders. (Yes, there are server-side filtering systems. I don't know any GUI clients with first-class support for them.) There's one address book, and one place for that annoying 'people I reply to go into my address book' setting, and my mail sig, and all those other stupid things you need to tune every time you get a new email client.
The Android GMail client is a perfect example of what a client looks like in this world. It talks to the (secret / private) GMail API, it does offline mail reading, and queues actions so you can archive / filter / whatever mails while offline and it'll push changes later. You can read and write mail. It doesn't try to do anything clever, because anything clever done on one client isn't reproduced on any other client. And if I don't have a client on my current computer for GMail, I can use a web browser, and still get all the features of the server. I use the web gmail interface for everything anyway, because it's better than any GUI client I've got.
GMail is a long way from being perfect. I'm not saying it's the Solution. Maybe you disagree with the auto-conversation threading, and there's the large nit that you're not allowed to write your own client on the GMail API (due to the Google terms of service, plus you'd have to reverse-engineer it anyway). But I believe that Brent's effort is never going to produce a truly great mail client because 'Uses IMAP' is one of his core requirements.