Touchdown is good for Exchange based email. I really like how it interfaces with the contact app, so I can touch a name on an email, go to their contact card, and from their easily find any events or emails from that person, as well as any notes I have for them. In general they seem to allow for a lot more customization then the stock email app.
The biggest negative for me is that there doesn't seem to be a way to use the keyboard in landscape to reply to an email, so I've been using the stock email app to reply to all messages so far, and just using Touchdown for reading.
I just installed K-9 for a personal account, but haven't had enough time to play with it yet to see how much I like it.