I've used Gmail via Exchange on my iOS 5 iPhone 4 for a while now. Recently, I upgraded to an iPhone 4S. However, I noticed that now, after I setup Exchange battery drain increases significantly (from 0.5% drain for 1 hour idle to 2-3% per hour). Setting up Gmail via the imap (default Gmail access) works better for battery. However, I can't live without push email, and I don't see any reason I should do without a feature I've come to enjoy for a number of years.