I discovered Caustic when it was featured in Google Play quite a while ago. I was completely new to FM synthesizers, and was overwhelmed by the interface. I disliked the sound of the demo song, so I experimented on it while I was learning Caustic. I worked on it here and there, then decided I had a decent-sounding piece. I bought Caustic and exported it.
Here it is:
Here's a shorter version: