Here's my response, and it echoes what others have said.
I'm a fan of Dave Asprey. A big fan. However, like anything, Bulletproof coffee is something you have to try if you're a regular coffee drinker. Those who are, like daily, it's very easy to incorporate and test. Like others, I'm always looking for a new edge to my morning/after coffee.
The stuff is legit. At least N=1. I feel great when I use MCT Oil + Butter. I also use Cinnamon + Stevia and blend it for a minute. The result is a fabulous, filling and creamy drink that can last a few hours (I generally make 32-40 oz at a time and carry it in a thermos).
I have a pretty demanding job, at least one that requires a cognitive function on-point most of the time. I've definitely seen benefits not only getting started in the day, but maintaining a high-level performance for a longer period of time. Maybe some of it is placebo, thinking I'm consuming this great stuff that should make me perform higher.
However, if this isn't enough for me to buy-in, I've tried a few other things with it. Using it DURING a heavy strength/Crossfit workout. I'm not sure if it's the MCT Oil (most have tried this as a pre-workout in the past) or the caffeine (or a combo), but it works. It's works great. After letting it cool, I've sipped on BPC for an hour and been able to complete things I've never done before. Again, maybe there is some natural progression going on here, but in-comparison to previous stuff, it's seemed to work better.
So in conclusion, maybe it's not magic, but for two things I take very seriously (work and fitness), I've seen serious benefits both physically and mentally since I started making BPC (or some personal variance) everyday.