Not there yet

This is promising app which emulates a 4-operator (aka four oscillator, but in FM you usually use at least one oscillator as a modulator) FM synthesizer like the DX-100 or TX-81Z.

The sound is nice and it has a nice set of Yamaha-style algorithms (i.e. ways of wiring up your carrier and modulator oscillators), and a variety of waveforms (i.e. more than must sine waves) give you additional flexibility. Effects include a basic delay/chorus and fixed lowpass filter which can be used to cut some FM grunge for soft sounds. Audio cut and paste and a simple step sequencer are also nice.

Unfortunately, the app is just not very usable because the sound keeps dropping out! Please keep working on it, because I would love to have a nice FM synth on the iPad!!

AvengerPod about DXi FM synthesizer, v3.3.1