There isn't any other than the occasional message when you save that tells you to buy the product. It's about as close to freeware as a paid product can get.

I do suggest people pay though, it's cheap for a one-time purchase. The only reason I've ever seen the message at all is because I spent months being too lazy to dig up the license key to send to my work email. (That should also say something about how little I was being bothered by the message too)