I have sold about 6000 licenses of the Game Engine since.
Impact was my bachelor thesis and I wanted to release it as open source initially. However, after I finished university I poured a whole lot of time into polishing and documenting it, so I thought "what the hell, I'm going to try to sell it". Surprisingly it worked. Impact pretty much financed all the other projects I've been working on since.
I believe I was at the right place at the right time. Apple just announced that they won't support Flash on iOS and Impact was one of the very few solutions to make (substantial) games in HTML5.
Impact has only received minor updates since its launch. I tried to start with a fresh new version a number of times, but always stopped short - I felt like I couldn't deliver anything worthy of the high expectations people had. Sales have died down slowly over the last two years, partly because Impact is somewhat outdated now and makes some things more complicated than neccessary and partly because there are now very strong Open and Closed Source alternatives in this space.
At the beginning of this year, I finally found some new perspective on what a "Impact 2.0" should deliver and most importantly, the motivation to implement it. It's currently my favorite project to work on. It's a complete rewrite. Simplifying everything with the insights I gathered over the past few years is humbling, but I love it.
My plan is to have 2.0 ready at the end of the year. I'll probably publish Impact 1.0 under GPL then.
Also, I'm excited for Impact 2.0