You are also supporting the dev which encourages more updates.
Within 10 minutes of setting up the free version, I went and bought it.
It is the only mail app that actually works for me.
My pop3, hotmail, Gmail - it receives and sends, no muss, no fuss.
I would always get errors when trying to send through my pop3 and had to resort to forwarding through Gmail; which while it worked was a kludge.
Since I bought the app in December I think it's been updated three times?
And the dev answers emails very quickly.
Short answer - yeah, it's worth the $17.99.