I didn't use the challenges for math practice. Err... I don't think. I'm not exactly sure what the challenges are. I did go into the math problem forum a few times on this site and answer some questions that people posted. But really, my feeling after taking the test is that people are practicing math problems that are different and more difficult than what is on the gmat. I think I could have done better if I had really gotten a number plugging strategy down for each DS question. If you look at the difficult math questions they give you, the majority are percents, geometery and exponents/prime factors. But I think since people feel those are less difficult than combinations/perm questions, that they study perm/combination question far too much. At most you will have one or two perm/combination questions on the GMAT.
Thats my 2 cents.