WoW has it's share of gimmick fights. I'm not a huge fan of that terminology because it is used as a kind of absolute description when in reality there is a spectrum or a sliding scale. Every encounter has mechanics that force you to play differently for that boss. Sometimes, however, these mechanics so deeply change how you are playing that you feel like you are playing a little mini-game rather than playing WoW.
Obviously vehicles fights are like this because you literally are playing a mini-game, but every other fight falls somewhere on the spectrum. I would describe Chimaeron as a very gimmicky fight, at least for healers. And the mini-game that it asks healers to play? Wrath of the Lich King!
Outside of the feud phase, Chimaeron is a return to the days when people had three health states - full, almost dead and dead. Added to that is the fact that every healer can return someone from almost dead to "full" in less than 1.5 seconds. Inside the feud phase Chimaeron is a massive AoE damage fest. It's Wrath of the Lich King healing split into two parts.
Heroic Chimaeron adds a third Wrath era healing mechanic to the mix - the dire necessity of a Discipline priest. When healing is impossible and everyone is constantly taking damage, Power Word: Shield is the only thing that can save an attempt from a particular kind of relatively minor mistake.
Chimaeron is not a great fight. I think gimmick fights rarely are. The problem with them is that usually heavily favour one class or another. Bringing a Blood Death Knight to Vezax allowed our guild to get the realm first hardmode kill despite being a 10-player guild. A party of one shaman, one holy priest, one hunter and seven holy paladins could have won the heroic Valrithia encounter in under 15 seconds - before any adds could even come into play.
Similarly, because self healing isn't really "priced" for dps classes - that is, some do it for free in amounts significant to Chimaeron and others don't - some dps classes are greatly superior to other for this fight. Saving a healer nearly 6000 mana every time you get spit on is a huge asset for early kills.
But despite not liking gimmick fights, I think Chimaeron was worth making. Gimmick fights are bad because they usually don't work out well. But if the developers never tried crazy ideas just because those ideas rarely work out then the game would be a lot more boring. For that reason, I think Chimaeron deserves a pass.