One e-mail standard that Windows Mobile has been missing since day one is IMAP idle--an advanced form of the IMAP standard.  The idle version is essentially push e-mail and it's very popular (Gmail officially supports it) as it is quick and is pretty good with battery life.

Fixup over at XDA has taken the open-source IMAP Pusher Service and made a handy WinMo client--one that also supports multiple IMAP Idle accounts.  He points out some interesting drawbacks in using Exchange Gmail too:

  1. Almost all mail servers now support IMAP idle, including Gmail, while AS only works with MS's proprietary Exchange server.
  2. Gmail's AS push is still not reliable enough to be practically usable. It does not let you use your own domain as reply email address. When you delete a gmail message in AS, it goes into Achieve instead of Trash.
  3. AS does not support multiple accounts. You can use both AS and xImapPusher at the same time on the same phone.

To setup, have your IMAP account setup already in Outlook; set it to check once daily.  Install the xImapPusher .cab to your device (soft-reset for good measure), run the 'Accounts Config' followed by running 'xImap Pusher' application.  You should then see your data-arrows activate and begin pushing your email.  You also need to have .NET CF 3.5 installed.

Downside is there is no scheduler, so you'll have to remember to turn it off (run 'xImap Pusher' again) if you don't want it checking.  Still, we've been using it for a few hours now it seems pretty solid and simple to us, so give it a shot.