I did all 25 challenges and just recently took the GMAT. I can say for sure that the challenges are much much harder than the GMAT. I got a Q50-95th%. Here are all of my challenge scores for you reference:
Test Correct Total % Correct Mean time msec Percentile
m00 3 5 60.0% 1m 47s 10724 63.5%
m01 30 36 83.3% 1m 55s 11529 95.4%
m02 33 36 91.7% 1m 36s 9666 97.8%
m03 32 37 86.5% 1m 48s 10836 92.5%
m04 25 37 67.6% 1m 38s 9825 55.3%
m05 31 37 83.8% 1m 54s 11472 94.7%
m06 23 34 67.7% 2m 04s 12484 75.9%
m07 30 36 83.3% 1m 57s 11754 90.5%
m08 23 35 65.7% 1m 58s 11834 81.8%
m09 27 35 77.1% 1m 07s 6781 92.6%
m10 26 37 70.3% 1m 25s 8526 75.7%
m11 28 37 75.7% 1m 55s 11529 76.7%
m12 25 36 69.4% 1m 53s 11336 73.3%
m13 29 37 78.4% 1m 17s 7786 88.0%
m14 28 37 75.7% 1m 33s 9395 81.8%
m15 26 37 70.3% 2m 01s 12151 66.7%
m16 26 37 70.3% 1m 45s 10600 46.2%
m17 28 37 75.7% 1m 39s 9999 50.0%
m18 27 37 73.0% 1m 40s 10053 60.0%
m19 30 37 81.1% 1m 27s 8783 70.6%
m20 31 37 83.8% 1m 48s 10847 47.1%
m21 28 37 75.7% 2m 01s 12127 69.2%
m22 33 37 89.2% 1m 32s 9252 78.6%
m23 33 37 89.2% 1m 26s 8617 91.7%
m24 32 37 86.5% 1m 33s 9371 100.0%
m25 31 37 83.8% 1m 26s 8665 78.3%