Regarding "less cool than writing your own engine" - I do not actually want to write my own engine. I'd like to make some significant changes to this one (throw out stuff, or organize it differently) "as I see fit", and re-implement it if there is a need for it, without having to care about breaking things or doing them differently. The problem is, if the result is good then it is hard to bring them back easily, I assume, because the code may have diverged too far.