The GMAT and I have become fast friends-- I am not a great standardized test taker. These tests have been instrumental instrumental in providing quality problems that are challenging. I will list my pro's and con's (these may not apply to most who only use it to study for one or two tests, and the second may just be an anomaly I felt I noticed)
Pro's
-Great step by step solutions including direct links to discussions of specific problems in the forum (this is by far the best I have seen out of anything else out there)
-Very challenging sets
-Good post-test stats analyses interfaces
-Substantial question bank compared to many, let alone all are high quality questions.. bar none, has the highest number of 600+ problems
-Relatively easy testing interface
Con's
-If you try to study again (like I have for the second year), it will keep your stats and there is no way to reset. I ended up starting a new account which BB was extremely helpful and quick to respond in this process
-I will add (and maybe others haven't seen this)-- do not get discouraged by the scores. I took several tests and had several in the 30s yet several 47+/-2. The last test I took right before one of my test days was abismal-- like 35 or something. I was freaking out. The next day I scored 47 on the real GMAT. I would take the grades you get on these with a grain of salt as my scores were all over the place on the tests yet I was always 45-48 when I took the real test. In my case, I missed 2 easier problems yet had similar if not higher than average percentages for 600+/700+ problems on that particular test-- I personally felt the pt reduction I saw in the score for these couple of problems was way too severe compared to what would happen on test day.
All in all, these tests are the best resource for improving your Math Scores. I'll also give a shout out to Math Revolution for the best resource I found for learning the materials through video instruction but you need the
GMAT club tests to apply what you learn in Math Rev. if you want to take a bunch of practice sets.