Well, I work for an email provider. Out of all the cases I've seen probably around 100, only 1 has had any issues with ActiveSync. Some have had to delete the account and recreate, but most went through without any issues.
The one account checks email fine, but for some reason the iPhone doesn't send the SOAP calls for sending emails. I'm at a loss on how to fix besides a factory reset at this point. Like I said though seems like a very isolated case.