I almost never agree with Trekkies! They usually pan "Star Trek III" and label it a disappointing follow-up to the classic "Wrath of Khan." But I just don't see anything wrong here. The Klingons are delightfully over-the-top villains, the effects and spaceship models are great (arguably the best in the series), and the theft of the Enterprise is a wonderful sequence loaded with humor and tension. DeForest Kelley gets some great material as the "possessed" McCoy, and Shatner's performance - slightly more understated than in the last film - is again rock solid.
So what's the problem? I suppose this movie has difficulties standing on its own; it relies heavily on knowledge of "Khan." But, such issues inevitably crop up when you're dealing with a long-running series of interconnected movies, and they don't matter much in terms of raw entertainment value. Some fans complain that nothing really happens in this film - it's just about getting Spock back and nothing else - but the death of David and the destruction of the Enterprise load it up with more than enough dramatic punch for me.
And, can you possibly imagine Picard stealing the Enterprise to go on a rescue mission? I can't. This movie's storyline captures exactly what makes the original crew so warm, funny, and rebellious...and so it's a good Trek movie, despite what the fans will tell you.