I use IMAP for sending and receiving. Works reliably.
You dont get push, the phone checks for updates on the imap server (google) periodically, so if ou have it set to check every 15minuntes, if you get a mail one minute after the phone synced you wont see it for 14 minutes. This is not a problem for me.

Ofcourse it is one more thing that wakes data and processor up periodically, but I dont think its one of the bigger battery drains in the n900.