I bought the Machzor on erev Rosh Hashana and looked through it briefly on Rosh Hashana, my review is in no way comprehensive.
The Machzor is an adaptation of the Artscroll Yom Kippur Machzor. The translation and the instructions are from Artscroll.
The Machzor adds (or replaces) the following to the regular Machzor:
1. A nice introduction by R' Shachter and R' Genack
2. A very extensive section on the hanhagos of the Rav, although I had seen many of these printed already (Nefesh Harav, MiPninei Harav, Mesora, etc.), there were hanhagos that I had never seen before. In any case, it is very nice to have all the Rav's hanhagos relating to tefilla and Yom Kippur in one place.
3. The commentary on the bottom of the Machzor is taken from the Rav's shiurim, articles etc. The commentary in many places is extensive. A lot of work was taken in preparing the commentary from all the sources.
The Machzor works as follows. On the top is the regular Artscroll Hebrew and English text, including the directions of what to do. Under that is a section where the Rav's hanhagos relating to the specific tefillos (of that page) are noted (there is an indication in the Machzor text that refer's to the Rav's hanhaga). On the bottom is the commentary culled from the Rav's writing's and shiurim.
The use of the Artscroll Machzor as a base is understandable. It has become the defacto Machzor used by people who want an English translation. In addition, to translate and typeset from scratch the Yom Kippur Machzor is a tremendous amount of work. However, in this case it is a bit problematic and takes away from things. In many cases, the text at the top and the instructions say one thing while the Rav's hanhaga (printed below) says something else entirely. The Rav changed the nusach of certain things, but in the text above, the Rav's change doesn't even appear as a variant reading. In fact, by the Avoda, the Artscroll machzor prints the regular nusach ashkenaz Avoda. The hanhaga on the bottom says that the Rav had problems with this version of the avoda because it contradicted the gemara and therefore he said the nusach sefard version of the Avoda. In other cases the instructions say one thing while the Rav's hanhaga says the opposite. It would have been much better if the text of the machzor followed the Rav with a variant reading of the common custom (like you see in the siddor of the Gra etc.).
Notwithstanding the above, if you consider yourself a student of the Rav or one of his Talmidim (e.g. R' Shachter etc.) it is worthwhile to get the Machzor. It does a very good job of pulling together both the Rav's hanhagos and the Rav's thoughts on the Yom Kippur davening from a whole host of sources and putting them into the proper place.
One final note, the Machzor at least in Israel is quite expensive, 193 shekel, about $45, I believe that even so it is a worthwhile investment.