I've done the butter in coffee, as well as butter + mct oil. I've never purchased their coffee though so i can't speak to that. As far as the butter and butter+mct, I certainly get a nice boost.
Speaking on just coffee and butter: The feeling I get from this is not surprising, and i don't think doing butter in coffee, or having coffee and butter separate is different. IE. I could have a cup of coffee and just suck on a stick of butter for the same effect. I just do coffee+butter on days I'm not doing breakfast for some reason (as it makes me feel full and energetic easily into the afternoon).
When adding in the MCT oil, I definitely get a really clean headed boost in focus...like energy behind the eyes. I don't know how to describe it other than that - kind of like some energy drinks, but without the crappy feelings that accompany those. Now - this could be placebo, but that would be fine with me.
With all that being said, I dont think any of it tops a good nights rest, a solid breakfast, and a regular fresh cup of coffee - but it certainly can help from time to time.
Speaking to the flavor - in concept it's not that different from something like a cappuccino. I enjoy espresso by itself, but I also enjoy a cappuccino because the combined flavors are a unique experience. Good butter in coffee is deliciously smooth - but of course it affects the flavor. Some combinations end up being better than others.