I think that the MGMAT guides are great! They are very thorough and I think they are better than the OG guides for strategy and content, however the OG guides are definitely the best for practice questions. If you are having a lot of trouble with guide 1, however, I would recommend that you go back and start at the foundations of GMAT quant (MGMAT book in the strategy guide set). This should help get you up to speed on the topics and probably make the rest of the set go more smoothly.
Hope this helps and goo luck!