But as someone pointed out in another subthread, it doesn't matter if Google is the best secured, least spammed provider in the world (and they may well be), if they turn around and leak your contacts lists. The comments in the big long buzz thread about peoples' client lists becoming aware of each other was pretty egregious.
One could make the case that hosting on Gmail has greater privacy in this respect, since if you send to any other Gmail address, the e-mail probably stays within the Google network.