I was coding a particle system for my home engine. I wanted it to be quite flexible and after few iterations I had literally a dozens of parameters and transform modes. And this was not only a problem about design, it was a major performance issue.
I gave a thought about using expression parser. In some other project I've been involved, I've seen muParser library. But this was a no-go, as I had thousands of particles every frame and muParser was just too slow. And no, I couldn't offload particle computations onto GPU as GPU is heavily loaded with rendering.
