The GMAT Club tests are the best test prep resource you can use aside from the official material from GMAC. I found the GMAT Club material to be challenging enough that I was constantly being pushed throughout my preparation, but not overbearingly difficult to the point it discouraged me. I have tried several different test prep materials, but I always found myself back here verifying my work on the forums. After a while, I figured I might as well try the GMAT Club tests, and I was very happy I did! It ended up being an integral part to my prep. I especially liked having the option to tailor my prep to exactly what I wanted to work on that day. Performing well on the difficult material gave me the confidence to perform to my best ability on test day!