As is well known, although this was the first 'Star Trek' episode to air in the U.S., it was actually the sixth episode to be filmed (not including original pilot 'The Cage'). Various theories have been presented over the years as to why this episode was picked to air first; personally I think that original second pilot 'Where No Man Has Gone Before' would have made a better premiere episode. That said, I still really like 'The Man Trap'; it is a mostly exciting story with an interesting creature. Some fans don't like this episode, and while there are a couple of flaws, I for one really enjoy it.
The 'salt vampire', as it is sometimes dubbed, is an interesting creature; I like the twist of how it appears different to every person, and it has a good costume for its true appearance. As some other reviewers have commented, its treatment is many a bit suspect compared with the handling of other unknown creatures in various other episodes, but I can't say it overly bothered me.
The episode also gives some good character development, most notably for Bones, but also with botanist Sulu, and a nice scene with Uhura trying to make conversation with Spock. (The character emphasis of the episode is one of the various suggested reasons that the episode was picked to air first).
There are a few things that let the episode down very slightly (in particular, Sulu's plant, which is quite clearly just a hand puppet), but all-in-all there is enough going on in this episode to make it a very fair entry. There would be much better episodes later on, but for the selected first outing, it's not too bad at all.