We also use gmail (well, we have a family domain name, which has the advantage that we could switch without needing to tell everyone new addresses, but gmail is what DS uses as a mail client). The one criticism I have of it is that you can't set up an automatic CC. So although I automatically get a copy of all DS's incoming mail, and although I know his password and can therefore check what he's sending whenever I like, I can't see in real time what he sends. It's more of an irritation than a problem, and the time is coming for us when his privacy would make it inappropriate anyway (aside: really not sure how to balance that against protecting a teen), but it's something to be aware of. There are probably other free providers that do offer automatic CC.
