Perhaps most people here find it so bad because they do not know how to efficiently use it. Given an average player on normal mode who follows all the Level suggestions when entering the worlds (i.e. is at Lvl50 when he enters The World That Never Was), most bosses require mostly button mashing (either X or O, depending on your region), with the occasional Curaga spell thrown in in between (which is way too wide-ranged and strong in KH2... it was nicer in KH1, where you actually had to cast it every few seconds if you were getting owned, but that's just my opinion), and maybe one or two moments where the player goes "OMG, mai draiv gaaage iz at Lvl3! Maibeh I shud switch to dat UBER PWNAGE VALOR FORM an do EVEN MOAR BATTON MESHING" (hope you can decipher this).
However, if you practice a bit, and learn how to use Anti-Form the way it is, you will find out that it can be used effectively. It just might require a change of tactics, i.e. from "spam Reflega until everything is nuked" to "run in, pull off a short combo, retreat and dodge, then attack again". Anti-Form is a challenge, and a sort of emergency brake for Form Change spammers. View it that way, and you'll sooner or later understand how to properly use it.
That said, I believe Anti-Form on Proud Mode (and Critical Mode on KH2:FM+) must be a total killer. For the player that is. Damage on those levels is already high, and doubling it by switching into Anti-Form is probably an instant death sentence, especially Saix's berserk mode. So yes, it's probably pretty much in the way there. On the other hand, if you have played on proud mode and then switch back to normal mode and find yourself in Anti-Form, you should have no problems dealing with it, as you should by then have a feeling for it.