I actually bought Kanzen N3 Grammar like a month before I took the N3. Barely even opened it, though, because I was still working through N4 grammar. My friend gifted me Tobira almost right after so I worked through that instead. Finished Tobira a few weeks ago and now I’m finally using Kanzen again.
Going through it, there is no way I would have stuck with this as my main resource from the get-go. The explanations are so short and dry. It’s only because I’m already familiar with the grammar points that it’s not a problem for now. But back then I’d be so lost.
That said, I think the test problems are invaluable and really are the reason I’m even bothering to go through it instead of jumping into N2 stuff. They really help me find my weaknesses and help differentiate the grammar points especially in the context of actual test-like questions.
I was also gifted Kanzen N1 & N2 Grammar + Reading so, even though they might not be my main mode of grammar learning, I might as well make the most of them.