Yeah, if you are really into aesthetics, and they are a dealbreaker, it's probably not for you. not sure how much nicer it can look though as it's a standard email client really.
I dunno… on Mavericks at least, I have to say it looks pretty good. The text-only buttons give it a nice minimalistic look. I have no idea how it looks on Yosemite though.
Try out the demo and see if it is for you.