The d20 Menace Manual is a good, solid product. To my mind, there are two major factors that keep it from being a great. I feel like it is a little thin for its cost (size to cost is a silly way to judge an RPG product but...), and some of the information is too campaign specific and so may not be usable for all purchasers as is. The Menace Manual has 3 major sections: Creatures, GM Characters, and Factions. At least a part of the latter two sections are not as portable as the other more widely useable portions of the book.
The d20 Menace Manual has 224 hardbound pages for $34.95 list (the 3rd edition Monster Manual was 224 pages for $19.95). So the Modern product is not that great price per page, but the mass market appeal is probably not the same as D&D so it is regretable but understandable that the cost is higher with the Modern product line. Even though number of pages or words is a arbitrary metric and it is silly to judge a book by its size, the book feels thin. However, the book's content is generally of high quality (probably a more important criteria). The Menace Manual doesn't really have player material, it is mainly targetted at GMs. I think it is good looking (but I like the d20 Modern dress- it follows that style). The art has highlights and low lights, but is mostly colour and is mostly of high calibre.
The Menace Manual has 85 pages of Creatures. The creatures are Monster Manual style monsters. Some would be great in D&D. The monsters are assorted by types and CR so you get a cross section of all. The fraal appear along with some Alternity and DarkMatter creatures converted to d20. There are a couple of templates in this section.
It has almost 50 pages of GM characters. As you know from the D20 Modern role playing game there are two types of NPC's (or GM characters). There are ordinaries, basically characters with average stats, HP with no feats, and heroic GM characters that are made just like the heroes or player characters. The first 25 pages are 'ordinaries' that compliment the ones in the d20 Modern Role Playing Game. The NPC's are divided into jobs and have CR 1, 5, 9 statblocks for each job. The second 25 or so pages are specific, named, heroic GM characters some allies, some foes. I find the ordinaries useful, the heroics not as useful. The generic ordinaries I can dress up anyway I like, the Heroics already have a certain flavor, and my game isn't really set for any of them. I can use them, but I have to adjust them heavily- which doesn't save me time as a GM. If you use one of the 3 proposed settings in the d20 Modern Roleplaying Game, you will probably be able to use more of the Heroic characters.
The next 80 pages are factions. This part is useful for a GM because of the templating. I don't feel the actual factions are that useful, but I'm using d20M as a cyberpunk setting. Again, if you use one of the 3 proposed setting from the in the d20 Modern Roleplaying Game, you will probably use more of the factions section. Each faction has details about organization, some sample characters, gather/info research charts if the players research the organization. The factions present some interesting concepts. The Hoffman institute makes an appearance and some other organizations have interesting flavor. I like the balance of crunch and fluff. I use the crunch of the sample characters and the template for a faction, the fluff is just inspirational for me as it doesn't fit my game.
Instead of an index, there is a creatures ranked by CR chart. I like the CR chart, but I also wish they would include a type chart (like in the beginning of the Fiend Folio). There are no appendix / index / concordance parts- the table of contents at the beginning is how you are supposed to find things.
The d20 Modern Menace Manual succeeds at the most important things for a GM supplement, it has time or preparation saving material and it has material to flavor your game. I'm using the Creature section in my D&D campaign and a few creatures in d20 Modern. The GM characters section saves me some time and I use many of the ordinaries. The factions section, I don't use much verbatim (except more GM characters), but the ideas and templating I found useful.
Overall: for a GM 4 out of 5 stars (B+) for a player 3 out of 5 (C maybe C-).