The notification bar says how many new messages you have. You follow the notification and get a list of messages. You read them, then they go away. You can press the star to flag. You can click reply to reply.
It's weird that you have to press menu and choose an option to send a message after you've composed it, but it's not really that bad of a UI choice.
Yup, I can never figure out why so many people recommend K-9 mail. I'm guessing from this discussion that it has more features for IMAP users, but for ActiveSync it has less, and the UI is as step backward, even more ugly and awkward than the built in Android Mail client, which IMHO is like the ugly duckling of the default Android apps.
I'm guessing that any ActiveSync features exist only by accident, from the original fork of the official Mail client. K-9 is an IMAP client, nothing else.
Are you sure? Because neither the description in the Market, nor the project description on the K9 Google Code project webpage say it's only an IMAP client. Here's the description from the K9mail Google Code Project webpage:
"K9 is an opensource email client based on the Email application shipped with the initial release of Android.
K9 is focused on making it easy to chew through large volumes of email. It's our hope that K9 leads to improvements in the core Android mail client. "
If they're intending it to be only an IMAP client, nothing more, they aren't doing a very good job communicating that.