After having used push email on devices for several years now across multiple devices (Windows Mobile 6 + Symbian 60) I can safely say email on the iPhone is, in my opinion, the best so far! Even comparing the iPhone to the newly released Blackberry Storm, the iPhone still works out cheaper as it works out of the box with Exchange 2003 & 2007 and no expensive additions are needed like Blackberry enterprise server.
So there's still a lot of things missing, no portrait email / read receipts / out of office from the handset (with Exchange 2007) and a way to centrally manage the handsets / push settings & policies (this is where BES is still much better.) Even with these omissions feedback is very positive, infact we've seen a large uptake of iPhones with our own clients. This doesn't just seem to be creative clients either with them slowly appearing amongst the financial and public sector also.
You could argue that the uptake amongst these sectors will be much slower until the phone can have policies enforced remotely and becomes more 'business like', although if over a third of the fortune 500 participated in the iPhone 2.0 software beta it shows there is a clear interest (before v2 the iPhone didn't support push exchange mail/calendar/contacts only POP/IMAP).