Ok. I bought into this out of curiosity. (worse still, I got them while they were full price, nearly $10, US)

Some sort of thunder league competition came and went that was available for me to watch, but I am not into watching.

The big (borderline HUGE) benefit I got was in the form of a random "gimme" that happened much like a battle trophy: a 100% bonus on research. It wasn't like the ones I'm used to getting, it was 100% for a WEEK (up to 900 missions!) Of course, the end result is that I have a LOT more planes and tanks available with no lions to buy them. I wish it could have been a bonus on lions, instead!

It may actually be cost-effective to buy into this.