Joe Kissell, in a follow-up to his original article on Mavericks and Gmail:

If you were holding off on upgrading to Mavericks because of the Mail problems, all I can say is that it’s safer now than it was at first. I can’t guarantee you a trouble-free experience, and without a doubt, some people upgrading from Mountain Lion will feel the new version of Mail is a distinct downgrade. It all depends on how you use Mail, and as we’ve seen, each person approaches it a bit differently from the next.

I’ll let you read Joe’s article to see what’s been fixed and which issues persist after Apple’s update. As I said earlier today, I’ve been fine with MailMate.