I took it once. It's just the same as every other SAT course out there. There are some tricks to the SAT, for example reading the passages only at critical parts and skimming the rest, but other than that, they can't come up with anything entirely new. The course just consisted of going over problems most of the time. I scored 2000 and 2030, even after I took the course in XDF (granted I wasn't paying attention 50% of the time. Ironically I did my PSAT after XDF and before my SATs and scored 222).
Dissatisfied with my score, I did it again. I took a crash course targeting Korean people for 8 days that consisted of a full SAT every morning and then going over problems/tricks and the test in the afternoon, and after 5 PM "independent study time", which I never attended. After lunch I'm there just because I had to be- I never listened to anything and don't recall consciously using tricks on the test that followed. I scored a 2260 (800 CR/770 M/690W), which is right in line with the PSAT score that I got before this course.
I concluded that I had it in me to get 2200+ all along, and what that course really helped me with was practice, practice, practice. I have ADHD so my performance, academically or on standardized tests, tends to fluctuate but for all it's worth XDF didn't help me at all. Maybe it was a rip off that I spent 5000 RMB on a crash course that did nothing but make me practice, but I wouldn't have practiced on my own and the 2260 probably got me into Carleton, as well as Wesleyan and the University of Hong Kong. Ain't complaining.