Linux

I've been using it at work, where my work email is in Gmail. I find it a good desktop email client, though even with its redesign, some things show their age. I also find that if you're using Google for your mail, there is still some stuff lacking. I blame Google more for that than Thunderbird though :).

So in short, I call it good enough (I like it way better than Apple's Mail and Calendar programs), but it is still bas a ways to go to be great.