I mainly used tests for quant practice and I liked it a lot. Questions are much more trickier that actual GMAT's ones but very clever so most of the questions test several areas. I also liked the explanations and the discussion under each of the questions. Stats are also very powerful (time spent, breakdown by correct/incorrect answer, timings for each question, comparison to others) while test's interface is similar to the real GMAT's one. I have not tried any verbal tests yest but quant practice actually boosted my score from 40-42 to 46-48. I also got rid of timing issue on official GMAT tests and finish quant section with around 5 spare minutes now (vs rushing at least 5 questions to make it on time).