I guess it depends on what your priority is. If you want a way to keep an eye on your inbox without the high battery hit and higher data transfer that you’d get from polling, then IMAP IDLE from a mobile device is a good answer. But no, you’re probably not gonna get updates as quickly as with a system that uses a BlackBerry-style notification backchannel, because you could end up waiting for the next time your phone tries to ping the gmail server.

Even allowing for this, it’s still pretty quick though – and the noop ping isn’t too expensive.

This all depends on whether you have a mobile client which has support for IMAP IDLE, which doesn’t seem to be too widespread yet. For example, if you’re on Windows Mobile, the default Outlook client supports IMAP, but not IMAP IDLE, so you’re left with having to manually Send/Receive to get updates or poll like you would have to with POP3 access anyway. Not very “push”. I think there are a few Symbian S60 devices which support IMAP IDLE out of the box though – I’ve seen people use gmail’s IMAP with the E71 and say good things about it. So for some people, it’s probably worth a try.