I tried them when my DD was 5, she was an excellent reader for her age and I was trying to transition her to chapter books. So many moms said their children loved MTH, so I thought I’d give them a whirl, and my DD hated them. They are very formulaic books, after the 3rd book my DD was done with them. I think its one of those things, YMMV. I was not impressed with the books, but for me it was just going to serve a short purpose, so I was willing to try, we had good literature going on in other areas of our home, but MTH didn’t work for us. I just went back to what I was doing, I continued to give my DD beautifully illustrated picture/story books (that were written at a higher level) than MTH. Visually that worked better for her, and then later I tried again, in a different way, to transition her to chapter books, and it worked.
So I would say it really depends on your child, your purpose in why you would like to use those books, etc.