The bloggers hardly cover the topic in this thread, or don't cover it well, I have to say it is under-rated method to buy miles. My guess is the bloggers don't have big tax payments. Just paid $.012 for a bunch of UA miles (I average about $.03/mile redemption, my home airport is really expensive) and SPG points for $.019/mile (this was more the end of an experiment so I could get a travel package), not convinced yet the SPG points were a good method.

Combining UA Club card with buying miles via tax payments and saver tickets is a huge win for me. Only downside is I don't have status because I fly miles all the time.