What kind of email server do you have? If it is Exchange, try TouchDown for Exchange (fully functional 30 day trial, then $20). I tested it against Motorola's Corporate Sync and against K-9 on a Droid X, K-9 was missing a few too many little features (this was in December).
Also, Exchange ActiveSync "push" email works by opening a connection to the server and listening for incoming traffic over an extended period of time, rather than polling the server at set intervals. I suspect Blackberry works that way as well. When I tested K-9, it did not have push for Exchange.
At the moment I'm still using HTC's email client. The things that annoy me the most are: cumbersome to change folders (e.g., to Draft and back to Inbox); absurd to have a huge long flat list of folders (rather than a hierarchy) when I want to move a message; and unable to read embedded Outlook messages (forwarded as an EML attachment).
The server is not an Exchange server. It's at a hosting company but it supports IMAP. It works fine for my Blackberry. I get emails seconds after it shows up in my inbox. Sent emails from my Blackberry are added to the Sent folder on the server. It works perfectly. That's why I have a hard time seeing an Android phone as a true business phone. It just isn't. Android phones are great for all the "fun" stuff but they are seriously lacking when it comes to actually getting business done.
I don't mean to put down the Android phones. It's just that people can't pretend they're something they're not. Some people really need to have phones they can work from. I'm also hoping I'm wrong and there's a solution that I don't' know about but it doesn't seem that way. Well, maybe in 2 years when I can renew my contract they'll have models that can do these basic things. Hopefully.
Richard