The included Email app has native Exchange ActiveSync support. IMHO it's pretty basic. It is perfect for those that just want to check their recent mail including any account folders, reply to those, compose mail to addresses that they can remember, and have the Read/Unread status synced. What it doesn't do is let you search your e-mail, no GAL, input just the recipient's username (recipient AD account username is supported in Windows Mobile), and no access to Public Folders. If you get a lot of mail, it will play the sound you designate for each message and can get really annoying fast. Android only has a basic sync schedule of intervals and doesn't do different sync schedules based on one's work and off hours.
Personally it works ok for me. I've missed some features from WM, but I don't do a lot of e-mail when I'm not bound to my cube. In a pinch, I can use OWA via the Browser app to find a specific e-mail; the browser is one area that WM has severely lacked.
If you need a better e-mail client for your Exchange account, there are 3rd party apps that can fill that need.