I dont own a HTC Touch Diamond or a Touch Pro. I own a HTC Touch and use Hosted Exchange as my primary email service provider. I use the direct-push technology in WM6 and sync with my exchange account. It works like a charm and I have yet to experience any problems with it. I get my email on my Touch within 10 seconds of receiving the email in my inbox.
Memory fades about Exchange 2003, however, I *think* Exchange direct-push is enabled by default on Exchange 2007. Someone please correct me if I am wrong.
HTC Touch (Elfin)
Both running Windows Mobile 6
'nuf said !