This is by far the best instrument from Synapse. I got the SynthPack from them and also use the Scorpion occasionally which is nice for bass sounds, and then there is the Plucked String which I barely use. The Hydra features multiple models, it can do both subtractive and fm-like synthesis. The filter and envelopes are always available, something which pure FM synths badly lack. Additionally there is a percussive mode for drum sounds. The drum sounds included are not too useful, but the mode still is for other synth sounds.
Version 4.0 had some stability issues. They released 4.1 shortly afterwards, that runs rock stable, did not crash on me once.