Caustic is a truly innovative app. In some ways similar to things like Gadget, but very different too. Also, it’s one of a very few cross platform apps with this level of complexity.
Caustic is unusual in that it started on Android and moved over to iOS and has been very well received on both platforms. Of course the latest iOS update has made a lot of people very happy too.
Personally I love the modular synth in Caustic. It’s awesome and really intuitive to use with great results.