I'm more with MMM than against on pretty much everything everyone has mentioned so far in this thread. I do think that he's a little more glib than necessary with some of the health stuff, but that's an unsurprising side effect of the rest of his views, which are a huge net positive given mainstream consumerism - the largest net positive I'm aware of, really. Well, ok, no regular dentist visits is a little horrifying, lol.
However, there is one thing I do have a huge issue with, and that's the house of cards 4% rule. He's not the only one pushing it, so he doesn't deserve all the blame, but he has so much reach that he has to be held to high standards. With the decade long bull market, and the creeping up of CAPE, this is getting more urgent: while it turned out that anyone who FIREd relying on 4% within a year or two of MMM's 4% posts was perfectly fine, the past couple of years are much more dangerous. I really wish he would realize it, and publish an updated withdrawal rate article.
I'm new to the forums, and I wouldn't be surprised if this has been the subject of many megathreads, so rather than going into the case against 4% in my own words, I'll just say: check out EarlyRetirementNow's Safe Withdrawal Rate series.
On the bright side, it's nice that my problem with MMM is neatly contained to one subject. If I'm getting someone into FIRE, I can tell them MMM is all they need for the lifestyle philosophy stuff, but to completely ignore all of his financial advice, and instead go to EarlyRetirementNow. Still doesn't make for a great pitch to tell someone about this font of life changing wonderful advice, and then add the caveat that the financial side is dangerously overoptimistic, though.