Man, I haven't been here in a long time, but this is a topic dear to me, so I'd like to comment on it from the perspective of a family that has used all 6 levels of MCT with success.
Those who say it's not a good choice for parent-teachers who are not the best in language arts themselves are probably right. It has a high level of presupposition of teacher knowledge and comfort. I am a professed language nerd. Although I had not the benefit of the best grammar *instruction* growing up, I had the advantage of hearing flawless grammar *usage* in my home, and of being exposed to a wide, wide variety of really good literature, children's and adult, in my own homeschooled experience. MCT gave me names and understanding of WHY things that are correct are, but nothing in it was new to me in terms of actual usage. So for ME, it was easy to jump into and use, but I realize my experiences are not the norm. OTOH, I have recommended it to several friends IRL with backgrounds far different from my own and they too have been using it with success. It's really the ultimate YMMV curriculum, I think.
As to whether it is appropriate for non-gifted learners, ie ordinary bright kids, I think it is. I used Voyage level this year with my ds13 in 8th grade and another student, a sophomore who had struggled with LA in the past. For him it provided a level of instruction in the grammar, vocab, and poetry that appropriate for him, and I was able to use the writing portion to good effect with him in combination with the LToW level 1. I think that the idea to keep in mind is that it's vital to use the level that is suited to your student, without regard to what grade it's "supposed" to be for. In the case of my student, it was just that he was not ready for Voyage level material before 10th grade. Good. Start there with it, and keep moving ahead. The important thing is not to have completed all the levels, but to find the student's level and begin instruction there.
But. But. But.
Yes, I'm a confident writer, and yes, I'm good at critique. But using MCT gave *me* language to use to help direct students toward beauty in their writing; I felt that the direction he gives is sufficient for that for me. This is likely not the case for everyone, and I see and appreciate that. However, and no offense to you, SWB, I couldn't have used WWE, WWS, or FLL if you'd paid me to. I found the approach to writing incoherent, and the scripted lessons stultifying. I love your history books, and I love the WTM approach, but we just approach language skills differently, I think. The oft-quoted maxim of "the best curriculum is the one that gets done" certainly applies here! ðŸ˜Š
Now, my dds17 have used all the levels except Island, and they have told me that they have found MCT valuable and easy to use. They are NOT LA types--they're heading into the sciences--but used one of the three secondary levels for each of their three years of high school. We had done the first two levels together, and they did the second three independently. They are not my best writers, but they are proficient enough. (They would not be my best writers, regardless, I think. They're just not interested in English the way I am; but one of them said wistfully on the way to church today, "I can't wait to start calculus. I miss Ms. S, and I just love math!" ðŸ˜‚) Given the people they are, proficient enough writers satisfies me.
The last thing I want to say about MCT is that I think his focus is on beauty in expression in a way that no other program's is, and it is that that I find most valuable and outstanding. Nothing else I have seen is as focused on choosing the very.best.words in your writing, on tightening up your prose till it sings. Beautiful expression is so important, and it can easily be lost through worry about the number of adjectives or variations in verbs of speaking. (This last is one of the ways in which I eliminate novels at the library--I open to a passage of dialogue and if there are more than two verbs of speaking on the page, I won't check it out. Life is too short to read bad prose.ðŸ˜›) I find that MCT increases awareness of really good writing by drawing attention to what makes it beautiful, and how the author achieves that goal.
This small treatise has taken me an hour to compose, and that'll have to do, but the TLDR: do please include MCT in your recommendations; it has many strengths which are unique to it, and it can be valuable for a wider audience than is sometimes supposed.