After I finished with the abomination that is Star Trek V I decided to watch Star Trek III for the first time in a long time. It holds up well.
For one, Shatner and Nimoy both look 50ish as opposed to the 60ish they do in 5, and although DeForest Kelley is starting to look old, it works for where the character is mentally in this film. Jimmy Doohan still looks like "movie Scotty" too and gets lots to do in this one, as he will in IV.
The Enterprise seems more scarred than it did when Khan was finished with it, and Robin Curtis as Saavik is awful, and a character-killer. (Saavik will get about 3 minutes of screen time in IV.)
Sure it's annoying that Admiral Morrow says the Enterprise is "20" years old - maybe since the Gary Mitchell-era paintjob (when they painted the railings red and removed the gooseneck speaker things) but it's clearly more like 40. However, the stealing of the Enterprise is fun and Christopher Lloyd and crew behave like real Klingons, not the mumbo-jumbo philosophical versions that we saw 78 years later hanging out with Picard.
The special effects are quite good...watching this film made me realize even more (if this is possible) that Star Trek V is a piece of crap.