If your organization allows you to connect to Exchange with activesync, then both Touchdown and Moxier are good. If you're limited to webdav connections, then both Moxier and Touchdown have problems with calendar functions. Especially if someone sends an update to a meeting, or moves the meeting.
For IMAP connections, I have found MailDroid to be the best of the bunch (I've tried Kaiten, K9, Melwin, etc...). The only problem I have with it, is that I need to accept a self-signed certificate for an SSL connection, and MailDroid is not allowing that for now. So I either have to use MailDroid across an open channel (yikes), or stick with the much crappier Kaiten, since Kaiten does allow me to manually accept the self-signed certificate. I am currently working (slowly) with the MailDroid dev to try and get the issue resolved.