I suppose I fall into the category of those who are passionate about TMP. To me, it /was/ Star Trek: Phase II. It was more of a sequel to the original series than TNG was in a lot of ways, and it was the most progressive, I feel, of all the Trek films. I liked the hard SF approach. I also liked how the characters were challenged in subtle ways; sure, the characters went through hell in some of the other films, but it was usually big and bombastic, or "out there": "You Klingon bastards killed my son!" or "Spock's ghost, I am possessed by it, ooga booga".
Also, it was the most genuine to some Star Trek ideals I feel, in that the Big Threat is not phasered or photon torpedoed to save the day, but communicated with, understood, and allowed to depart in peace - even after doing some damage and hurting some people. Spock's journey in the film is, IMO, the best use of the character outside of some of the deeper Trek novels. If anything, the story is the most critical turning point in Spock's entire life: it's the place where he gains true wisdom in addition to intelligence, and stops fighting against who and what he is. It was also a good use of Kirk. I felt it displayed Kirk's genuine intelligence and insight, rather than focus on solely on his passion and inventive but reckless talent for destabilizing situations in order to make things happen his way.
There's a few flaws and outright goofy things, and some of the visual design doesn't come together. Plus: what the hell was the point of the transporter accident scene as a way to get rid of the science officer and leave a void for Spock to fill? It seemed more like somebody just dying to play with the oft-imagined but never shown fatal teleportation accident.
But it remains one of the few films I can watch and feel I'm actually watching science fiction, and not cowboys in spaceships with laser guns. (I say this as someone who likes cowboys in spaceships with laser guns in measured doses.)