Tried K-9.. it had a couple of problems that meant I couldn't use it
1. Being based on the Android 1.6 mailer, means it looks bloody awful (to the point of unusability) on a tablet, and pretty awful on even a Gingerbread phone, compared to the stock mailer (which is getting quite nice these days).
2. They get imap folders wrong - they download from the first message, not the last. So my mailing list archives with 100,000 emails in would take several years to setup in K9, at 25 messages a time, with me hitting 'more' constantly. This is a complete showstopper.
3. The 'sync last x weeks' feature is utterly broken, and produces only blank folders (probably due to issue 2... it's checking the first 25 messages, deciding there's nothing that fits, and ignoring the entire folder).
I'm just waiting for a port of thunderbird.. meanwhile the stock Honeycomb email client is pretty good now, and hopefully the ICS one will be for the phones.