Boy, was I wrong. Like, dead wrong. Not about the players going for it; I have completionists in my group and that was to be expected. No, I was surprised by how much I got out of it, and how well the story segued into my overall campaign. It was almost more an exercise in improv than a full scenario... and wow, did everyone love it to bits. I had characters duping a skeevy car salesman into giving them a usable car to get to Eldora (in the mountains, and they needed a car big enough to hold them all). I had characters dissecting a dead Mi-go before it decomposed and gaining some Cthulhu Mythos in the process. I had characters so in character it hurt, and a lot of sympathy garnered for poor ol' Harny (one PC had a mentally retarded brother they cared for, so Harny struck them particularly hard). There were fight scenes with the Mi-go galore after people went nuts seeing them the first time - cars being rammed into the Mi-go, frozen engines with the ice guns, massive pain and near death on the mountain... it was a hell of a battle, with memorable characters and great moments. Talk about a hell of a game! Everyone loved it and it ended up being one of the more memorable campaigns I've done, and I was so glad it proved my initial suspicions wrong. Never judge a book by its cover, and never assume that good planning and improv cannot absolutely make or break a scenario!
That said, the scenario itself is flawed. I did have to do a lot of fleshing out to make it viable and interesting, and that means "The Madman" really needs to be something that's inserted into an ongoing campaign as a side-quest. It's ideal for a breather episode and easy to make more or less difficult, plus everyone always loves Mi-go. The clues are too easy to obtain, while the battles can quickly become too hard if you don't pace things properly. It can be dangerous and even deadly - especially with the 28% chance or so that Ithaqua will be summoned while the players are there. There is not a lot of opportunity to scene-set or give a good mood for horror unless you are fleshing it out beforehand, the Investigators have Mi-go experience, or you make Harny Rooger some creepy Boo Radley type, making this scenario a bit more on the survival horror-action pulpy end of things. The battles are fun, fast, furious, and interesting allowing for actual threats and the possibility of death, although this greatly depends on the Keeper.
Make sure your PCs are well-armed, and if you feel extra nice, let them maybe have use of an ice gun or two. Just make sure you don't let them overuse the weapons, or the rest of your campaign will be easily ruined - after all, no self-respecting major Mythos beastie would simply take being frozen lying down, would they? Especially not something big, nimble, and quick. My suggestion - absolutely nuke the power of the ice guns in terms of range, speed, etc., or give them way fewer charges, a higher malfunction rate, etc. If the players want a powerful weapon and it suits you to let them have it, go for it - but don't let them have it for free. And remember, they probably scavenged them off of Mi-go corpses, Mi-go who were previously using the guns, so don't forget to subtract from the total number of charges left for that!
All in all, you will get out whatever you put into "The Madman". It's good for starter groups, but really does work best slotted into a long campaign since it's short, easy, and simple to plan for. The Keeper has a lot of work cut out for them fleshing out the scenario, and that really is for the best with this one. It's really too bland and hodge-podge to stand on its own. Have plans for if Ithaqua is summoned, how the Mi-go are doing what they do, why they are doing what they do, how much humans know, etc. Have ideas for making the characters memorable, and they will be. If you don't, this scenario will likely fall very flat, very fast. 6.5/10 - Just the right kind of scenario for action-minded people and an easy breather, but it's just so dull without significant Keeper work.