I'm biased as I was involved in the concept of Antidote, but I really like it a lot. I understand that it's not for everyone but there certainly isn't any of the standard Reason synths that do stereo unison oscillators, I also think that the filters are of a good flavor and not rehashes of the one's already available in Reason (the Antidote filter is based on cascaded OTA type, found in many Roland synths, there's also a model of the diode ladder filter found in the TB-303 which I like, esp to play polyphonically.
Also I think that Antidote has a good balance between tweak-ability and user-friendliness, imho it has a good flow, just like Reason's own synths. Finally, Antidote has some really high quality FX that imho are worth the money alone.
There's an exciting update of Antidote around the corner that will expand it's usability a little bit more, I can't really say more than that atm.
Sorry if this came off as a sales pitch, like I said in the beginning, I'm emotionally involved in this synth due to both the concept and contribution in sound design, but for me, Antidote filled a hole in the Reason line up. For others, I'm sure that it is Predator or Viking that fills that hole. And for some, the Reason synths are sufficient.