John Updike reviews this new edition in the Nov 6 New Yorker, which is available online and well worth looking up. With 100 pages to go, Updike tired of the "irritable sniping from the sidelines" and switched to the standard Library of America edition.
A few months ago I reviewed the Penguin edition of Uncle Tom's Cabin here in Amazon. I suggested that if you decide to read the novel, skip the Introduction until you are done reading, because it gives away several plot points that you are better off encountering for yourself directly.
The same applies to this new annotated edition I think. The novel is not so difficult that you can't simply read it through on your own. I suggest doing that first, in a standard edition, then going through this edition. Otherwise you are having only a mediated experience of the work. In other words, let the work stand or fall on its own merits first, before exposing yourself to the opinions of others about it.
Having read the standard edition earlier I then read this annotated edition "inside out". That is, I read the introductory chapters and the annotations themselves straight through and used Stowe's text as the reference. This is a better approach I think than trying to read the text for the first time with the annotations nearby, where they do intrude and interrupt the flow of the story.
When reading the annotations this way though you do notice the inconsistency in voice that Updike mentions. Most are carefully neutral but you get an occasional first-person remark like "I confess my eyes glazed over" (gee that's helpful), then "again, our eyes glaze over" or "I recall Baldwin's...". Or "I am close to turning the page." then "...bore us silly", in the same annotation. As if the two editors read, and experienced eye-glaze, in unison? Since there seems to be two distinct voices at play it would have been useful for each annotation to have been initialed by its author, Gates or Robbins. I started trying to guess which editor wrote which annotation--I suspect Robbins provided the majority of the historical background while Gates did the Baldwins, the "I"s, and the trendier ones ("To the modern reader, Adolph is unmistakably 'metrosexual'"). This disparity in tone is also obvious between Gates' public interview (Boston Globe, Nov 12) in which he too-casually terms the work racist, and the less judgmental and more nuanced approach of the majority of the annotations themselves.
Getting past that though the annotations contain a wealth of useful background. The Biblical references, the distinctions among the slaves, the nuances of hypocrisy, the literary conventions, the sheer mechanics of the business, the conventional wisdom of the time about the races, all are excellent and thorough.
So, if you are going to read Uncle Tom's Cabin, do so first, then get this edition. It's an indispensable addition to the work.