I can't help but think they have given up on the Mac client. It's been at version 0.3.14 for an unbelievably long amount of time--so long that I've just gone back to Mac Mail.
It's nice but has many frustrations in the UI - especially when dealing with large message threads.
There are a number of interface gripes I have, and it sometimes consumes about 1.5GB of RAM, but overall it's the best OSX email client I've used (that's being actively maintained).