Same here. It's the only one that I've tried that actually syncs correctly with gmail and handles multiple accounts well.

It seems like Spark doesn't support "labels"? Only inboxes?

I also use Spark, I love everything except their "Compose a new email" button. To me personally it is a bit hidden. But as I mostly respond to mails, it's not that big of a problem to me. Btw tags never worked for me anyway, but Spark has cool functions regarding bringing the really important (personal) mails to your attention. Also, their function not to jump to the next mail when you finish reading one is brilliant. I hate accidentally opening a mail I don't intend to read at the moment and then having to mark it as unread.