How easy or how hard it is to learn German depends not only on your ability and the amount of effort you're willing to put in, but also what language you come to it from. If your language is Latinate, you're already used to dealing with genders, which makes cases specifically and grammar generally easier. If your first language is English, at some level you will always be perplexed that a spoon or table have a gender.
Grammar is a granular detail of language acquisition that never really gets perfected, but that's why it needs to be drilled and drilled again. Anyone can learn to ask how much a cab fare is going to be, or where the bathroom is. The skills to take you to the level where native speakers mistake you for a native (at least in written communication) are the kinds that "Intermediate German Grammar" treats at some length. It's not exhaustive (some of the other books in this series are thicker) but it is good, and because it is a reasonably priced workbook, you can toss it after you've finished and buy another one.
As a final note, with a lot of these books you will see people complaining that the authors themselves make some mistakes. This can occasionally happen in textbooks (not even McGraw Hill is perfect, despite having a million years in the scholastics industry) but most of these gripes can be put down to regionalism and ongoing debates between linguists and orthographers. The highest echelons of German Linguistics cannot agree on whether the Esset (that "ß" thing) should still be used. Your high school teacher or college professor has their own opinion, and will many times present their opinion as an ironclad rule or the "right" way to do things. But believe it or not they, like me, you, and McGraw Hill, can also be wrong.
I personally think this Swick guy is getting it right, and I can be as churlish as any other joyless pedant writing reviews at 3 a.m. Highest recommendation.