I'm going to start off by mentioning the fact that Mythbusters, back when Grant Imhara (who plays Sulu in Star Trek Continues) was still on the show, examined a key element of the story here. Without saying too much, let's just say that they found some problems with it.
Secondly, I have seen this episode before, but very little of it was fresh in my memory; it loses little on a second watching.
This episode starts hitting some classic beats early; the three leads are well-characterised (this is another strong Kirk story) and it's not long after we beam down that a red shirt purchases the farm.
There's a lot of close proximity explosions that you would never get away with today (indeed, slightly reckless use of explosives is a common theme in this era of science-fiction); indeed all three of the regulars apparently ended up with tinnitus as a result.
We then get a rather good spaceship chase (I was almost expecting Scotty to say "She cannae take it, Captain!", although I don't think he ever quite said that line) and then the main 'arena', taking us to Kirk's Rock for the second time this season. The battle of strength and wits is thrilling as well as engaging, although I do wonder where the cameras are that allow the crew to see it on the main screen back on the bridge.
This is a great episode; Kirk in particular demonstrates the whole UFP creed in this. If the Prime Directive is about non-interference, the Secondary Directive is about defending yourselves, but not using violence unnecessarily - as he demonstrates in the climax. It's proved to be the right decision.
The Gorn costume is slightly unconvincing (it was probably uncomfortable for the actor in it) and there is also a fairly big plot hole in all of this; if you intend to lure a ship into a trap, shouldn't you bring enough resources with you to make sure that you can actually close the trap? Also, it's difficult to deter invaders without allowing someone to send a message back.
Quintessential Trek (and oft homaged); if it wasn't for the slightly dodgy Gorn outfit, I'd be giving this a 10.