I would not say under rated at all. Its my second favorite TOS movie behind STII.
I totally disagree with statements that Kirk was racist in STVI. The Klingon's killed his son, said there would be no peace till he was dead. They were his mortal enemy who had attempted to kill him on numerous occasions. So the fact that he does not like them is not a racist statement.
Some of the others did look bad. McCoy did look a little stupid as a doctor but not a lot and he had been drinking. Ohura not being able to speak Klingon was odd. I know its an odd statement but Spock is justified in his actions, he does hurt Valeris a little but to stop a interstellar war that might kill millions. I mean the needs of the many...?