I got my code for HEY yesterday and have been having a play with the Android app. I have to say that it is very slick and we'll designed.
Will I be switching to HEY myself? Most likely not, I just couldn't justify the cost and would need them to implement custom domains first, but it's still good to have a look at what they're doing.
Saying that, my host seems to be having issues with their mail servers today so you never know