Rated 5 out of 5 stars
Haven't needed to use the buttons yet, but the thread view has already saved me a lot of time checking emails in multiple folders.
I've been trying to get Thunderbird to work in the same capacity as Google Apps and this was the last add-on I needed. It allows me to see when some one replys to an email that goes to one of email lists that we use and is filtered into another folder so I don't double up on work done, miss an email, etc.
I couldn't recommend this any higher to people trying to move away from webmail into Thunderbird.