Mmmm finally had time yo watch that episode for the first time.
The titular doomsday machine robot alien thing was a cool concept and design (though I can't say anything about the effects I saw the remastered version) I admit I have a thing for gigantic space creatures/constructs so that definitely was a plus.
The guest actor did a good job at playing a crazy person, I have heard from some actor friends that it is not that easy and the makeup department did a great job at making him look run-down.
The episode did start quite exiting as well. However I lost quite a bit of interest when all that military protocol showed up and the episode became a "who has the bigger penis" contest between Kirk and Matt for a while.
And then it kinda lost me completely when upon hearing that the doomsday machine was headed for Rigel Colony and the "most densely populated region of our galaxy!" Kirk and Spock resolved to basically run away and tell Starfleet Headquarters...I mean what

?
Sorry my sympathies were with Commodore Crazy-Pants, their duty is to protect Federation Citizens -> Rigel is a Federation colony, they have to do everything in their power to protect them. That is my sympathies
would lie with him if I wasn't pretty sure that he didn't give a rats-ass about the colony and just wanted revenge.
Kirk however didn't even pay lip-service to saving the colony at any point in the episode, he only cared about "his ship" and then to avenge his friend. *slow clap*
Well, that and the Cold War anlogy was a bit heavy handed.
It was not bad, just a bit weird in parts. It's definitely one of the better TOS episodes, especially the cool concept, but personally I enjoyed "Balance of Terror" and the "Enterprise Incident" more.