I bought the Kaplan Quiz Bank about threeish weeks ago and have completed about 73% of the questions. I've done most of the medium and hard questions so the remaining 27% consists of easy questions. I must say that I'm pretty satisfied with the Q Banks.
A lot of people are saying that the qbank difficulty level is lower than say Manhattan and
GMAT Club test. This is definitely true but I found the quant questions were very similar to actual gmat questions in that they were not hard per se, but elegantly tricky. So over the course of doing these problems I got better at spotting common traps. You may use the
MGMAT and
GMAT Club tests to improve your quant "strength" but you may want to use the Kaplan quant questions to improve your "agility". However, I do feel that if you're already scoring a 45 and above in quant then the Kaplan quant section may be less helpful but if you're trying to get to 45, then you definitely will. The verbal section, is a different beast all together. The hard questions are hard and tricky but once again, they feel very gmat like. I think anyone at any level will find the verbal questions helpful.
While the questions make for good practice, I wish Kaplan had refined the quant and SC question categories. For instance, while creating a drill/test you can have the option of selecting question types but these categories are too broad in my opinion. Consider the quant categories: Algebra, Arithmetic, Geometry, Number Properties, Other, Properties of Sets, Proportions. The CR and RC categories are all good but I would have liked to see more categories in SC: Comparisons, Modification, Other, Parallelism, Pronoun, Usage, Verbs.
All said and done, the qbank worked for me. Will it be useful to you? That depends on where you are performance wise and what your goals are. Plus the GMAT Club discount is an added incentive to get it; at circa 10 cents a question it's not a bad deal at all.