Yes. It is worth it. Simply put, Maildroid is faster and better laid out than K9. K9 is not bad by any means, but Maildroid is better for several reasons. The pro version will stay close to the free version in features, but not having ads will speed up the app even more! "10 fold" according to the developer. Two things don't get commented on in various forums about Maildroid:
1) YAHOO MAIL!!!, gmail, and hotmail, etc. are all PUSHED to your phone...just like blackberry. Not "pull every 5 minutes" but an immediate push! My biggest issue with other android email apps was working with Yahoo mail...this one rocks! Plus, its faster than K9.
2) Communication with the DEVELOPER! Try the free app. If you wish their were a feature, email the developer! You can contact him REALLY easily and he responds within like an hour and answers all your questions directly. He doesn't brush you off. He doesn't try to push his Pro version AT ALL either which took me by surprise when going back and forth with him. He wants people to try the free app...get to like it and if they wanna support "the cause" and have no ads, then go ahead and go pro. If you email the guy, he listens and takes note of what you "wish" the app could do, and actually implements them. My one big thing that I felt was missing from Maildroid was a unread email widget...and lo and behold, the developer confirmed that hes working on it right now. Look, the guy even responded to this forum above "maildroiddev"...he's actively out there looking to help people and make this app continually better.
Fact is K9 had a huge head start on Maildroid so there are more customization options...but Maildroid is already faster, looks better, and handles email like I was used to on my blackberry for the last 3 years and in some ways, even better. That, and the developers accessibility and eagerness to continually make it better, is why I think it is definitely worth the 17.99 to upgrade to pro.
FYI: I spent hours looking through forums on a good email app for android and had I read a post like this one, I would've made the decision to try Maildroid sooner. Hopefully this helps others out there either having trouble saying goodbye to Blackberry, or not being totally happy with their current android email app.