I use Sparrow and I agree. It manages to make communication by email so lightweight and fun again, but it lacks the stream that Fluent has. I'm very excited to explore Fluent as well.

I actually purchased Sparrow, but I don't use it.

It was slow and had some UX quirks/bugs.

For a time I fired it up every once in a while as an email-backup solution, but I got lazy so I don't even do that now.

Eg. I usually have Gmail open in other browsers in Windows even if I run Sparrow on the Mac, and it takes several minutes for Sparrow to notice that a message has already been read, which is super annoying.