I ordered a mystery bag a couple of years ago. I am desperately trying to remember correctly, but I thought that you used to be able to order mystery bags by yarn content...I could be wrong. However, we were living in Germany at the time so as far as it being slow, well everything was slow so I can't be for sure about that.
All that being said, I did get a bag of beautiful yarn, I'm pretty sure it was all wool... I do remember several skeins Plymouth worsted which have since been made into felted bags. Might have all been Plymouth...but it was quality yarn. No acrylic at all.
I do think the value outweighs the wait. I've learned to be a bit more patient for mail after living overseas.