I took one MGMAT CAT seriously, and I scored a 660 on it. Took the actual test a week later and scored a 720. Felt like I could have done better with more practice.
For what it's worth:
MGMAT's sentence correction book was very helpful. The reading comp and critical reasoning methods didn't help me out too much (maybe that's just me though).
All of the math books by MGMAT were invaluable- I was an engineer in undergrad, but I was struggling with high school algebra when I started studying for the test (five years after finishing undergrad). MGMAT's math books were a thorough reintroduction.
Thanks and good luck!