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.
I am having performance issues. My CPU ramps up to 50% when i hover over the mails and move the mouse up and down.
I do not have the most recent PC, but my i5 2500k @4.6GHz still can handle modern games so an email client should run better imo.