I tried them all (not joking: I buy every seriously-looking E-mail app). They all failed in one way or another, the biggest problem usually being speed.
My #2 is Spark, which I actually use on iOS, but on Mac it's a bit too slow, plus it actually failed to send an E-mail without notifying me or returning the E-mail (I did report this as a bug), which I consider a showstopper. So Mail.app it is.
Uhm, I think there are some great 3rd party apps out there that are much more powerful. But I concur that an actual email client is the way to go, especially as soon as you have more than one address.