Full disclosure: I teach Greek Syntax and advanced seminary Greek, but not 1st year Greek Grammar. I have, however, taught beginning Greek in the past and also tutored students in Greek.
400,000 purchaser's can't all be wrong! I think that this would be a fair observation regarding William D. Mounce's Basics of Biblical Greek Grammar, 4th ed. (Grand Rapids, MI: Zondervan Academic, 2019). It's been approximately 20 years since I first cracked open my copy of Mounce's 1st edition back in my Maranatha days; it served me well enough back then, and it has continued to serve other students well through three more iterations.
Now, here's the thing about 1st year Greek grammars: (1.) Theres a ton of them out there, and (2.) like sports teams, everybody has their favorite, which they can defend vociferously because . . . well, just because. What I'm trying to say is, there's a level of subjectivity involved with this kind of analysis, and the ultimate bottom line is, "Did the grammar help you read your Greek New Testament?" All other issues are secondary.
From that perspective, Mounce's grammar is a raging success, but then so are most grammars with a halfway competent teacher and a group of students who are not forced into the class against their will. What sets apart Mounce, to a certain degree, is (1.) a heavy focus on memorization and paradigms, (2.) a straightforward "nouns-to-verbs" approach in progression, and (3.) an effective integration with online material such as "FlashWorks."
The first two points can be somewhat controversial. There is nothing "fancy" about Mounce's pedagogical perspective, and I'm actually quite ok with that (many may wish to look elsewhere for "total immersion" methods, etc.) The paradigm layout, in my opinion, is effective enough, though intelligent teachers may wish to adapt them somewhat (it's easy enough to make one's own charts on MS Word).
As to Mounce's decision to complete nouns, adjectives, and pronouns before even dipping the toe into present indicative, this stands in stark contrast with many other grammars these days, including that of my own Doktorvater David Alan Black, who introduces verbs on chapter 2, and Stanley Porter, who introduces 1st Aorist verbs in chapter 4 (and before present tense! Which, to be fair, from his linguistic perspective actually makes sense). I have mixed feelings about that, but naturally the inventive professor is free to adjust the schedule in which his or her class tackles those chapters, anyways.
The third point is an ongoing, cumulative improvement since the first edition, and the range of possibilities inherent within "FlashWorks" is incredible, for those willing to get their hands a bit dirty (I will confess, however, that a couple aspects of the Hebrew vocab version of FlashWorks drives me nuts; I haven't used the Greek side as much, and not at all as a teacher, so I can't comment too much on it).
Now, one point of critique (and this critique applies equally well to other grammars). The discussion of "aspect" (e.g., page 155) is, I believe, potentially misleading. Mounce, like others, conflates "aspect" with "aktionsart" (at least in my opinion), which has the potential to cause confusion for students going on to deeper study. In addition, since we are using the "imperfect" label for a tense (ch. 21), it seems needlessly confusing to use "imperfective" as an aspectual label (ch. 27) when referring to the present tense, when, in fact, we're really talking about aktionsart anyways. There's got to be a better way. (And, again, the Greek verbal system is probably the most controversial aspect of the Koine language in the NT, so good people may differ).
Overall, though, Mounce's Grammar is a significant improvement over previous editions of what has already been a top-tier introduction to biblical Greek. The book looks better than the oddly-tall 3rd edition (which wasn't bad, really, just a bit cumbersome), the layout of the chapters is better (e.g., the "halftime review"), and it integrates well with the free "FlashWorks" software. Kudos to Mounce and Zondervan Academic for their excellent work!