I purchased GMAT club test a few months ago and no doubt they are incredible. Their Quant CATs are of high quality but i am not sure about the quality of verbal CATs as i felt like there are a lot of SC and RC questions which are not like questions in GMATprep and also their algorithm is quite tough as i found. However their Quant test are no double one of the best tests. Although they are quite tougher as compared to real test but though these quant tests improve your confidence, your skills and a lot more.
To keep yourself on track, you can take these tests one each day or on alternate days.
Happy GMAT study :)