The Tomb of Annihilation Companion is a well-written, very extensive addition to the adventure content, with beautiful, original art to accompany it. For a novice DM, the information is presented so that it could be run without any preparation.
However, because ToA companion is presented in such a way for novice DMs, it really restricts how more experienced DMs can utilize the material. I don’t, personally, love the style of much of the content (I prefer lower magic/less “gamey” mechanics), like the Water Tubes or the expanded Dinosaur Race. The Jungle Travel and Montage sections completely negate the hex crawl that is essentially the main focus of the adventure. The formatting promotes a very linear narrative, without many "branch points" for character agency. Etc, etc.
Normally, this would not affect my rating of the content. D&D is such a highly subjective endeavor, to rate something poorly because it doesn’t mesh with my personal style wouldn’t be very helpful for those looking to purchase the content. In fact, the only thing that would need to change in order for me to change my rating from a 3 to a 5 is to make the PDF able to copy and paste content. That way I can utilize the content in a way that’s best for me. As is, I will have to invest A LOT of time retyping and formatting it for my own use.
If you're a new DM, this is a great resource. If you're an experienced DM looking to filch, it is much less helpful.