So, it appears another downside to Mustachianism is finding out that you're not as Mustachian as others!
I actually bought a new car before I found out about Mustachianism, and although I probably wouldn't now, I can understand why I did it. I bought a Honda Odyssey new that had been on the lot for almost a year, and I paid about $500 over what I believed the dealer invoice was (based on Consumer Reports, etc.). We kept our last Honda Accord (also bought new) 10 years before that, and we figure we'll keep this one at least as long. And it's our only car, so we ride in together, or I take Metro when we have to go separately.
These forums are tough because there are almost always people who are even better with the financial decisions, and it can make it tough to feel like you're doing a good job even if you've made vast improvements, if you haven't nailed it perfectly. It's probably like someone being excited that they lost 10 pounds, and pointing out they're still not a model.
Ultimately, I think the information here is great, and the level of enthusiasm is like having a workout partner to keep you going on days you're not really feeling it. But I also think it's important to note that progress can sometimes be celebrated, even if there's still room for improvement.
So an upside of Mustachianism is securing your financial future. A downside might be risk of feeling depressed that you're not already a super-Mustachian!