The native mail client in Windows Mobile 6.5 is probably the most powerful Exchange client out there. Windows Phone 7 did lose some functionality that WinMo 6.5 had. Specifcially, there are 3 things that I miss most:
* Can't read DRM protected email on WP7 (can on WM 6.5)
* Can't search the corporate directory on WP7 (can on WM 6.5)
* No "remove from calendar" button when you receive a meeting cancellation (there is one on WM 6.5)
However, WP7 has a really cool "I'm running late" button that sends an email to a meeting organizer that you're running late for the meeting. I've actually used that one
All of that said, there's a WP7 update scheduled for this month (in fact it could even be announced at CES this week), so it would be worth seeing if the update contained any new functionality to the mail app.