Mordenkainen's Magnificent Emporium fulfils a need that D&D 4e has had since its inception: cursed items. While I could go on and on about how it has tons of really great new weapons, weapon powers, weapon feats, magical items, and flavor text, what you're really buying it for are the cursed items. I suppose I should back it up just a little bit. MME is presented, loosely, in the fom of a book written, but then later redacted and then destroyed, by the archmage Mordenkainen. He adds in some nods to his archmage buddies (including Tenser, of Tenser's Floating Disc fame, and Bigby, as in Bigby's Grasping Hand), but mostly pontificates about himself. It's a nice stylistic flourish that makes the book more fun to read, and also is an excuse for the authors to have a bit of fun in writing it. All throughout the book, there are little sidebars with knocks about certain items by Mordenkainen, which, for the most part, are pretty entertaining, and each chapter is introduced with a longer monologue about the topic in Mordenkainen's distinctive style.
The book does have some good mundane and magical items in it. It unlocks the "superior" training feat, as well as specific weapon-based special attacks utilizing superior weapons training (and superior weapons); along those lines was something I suggested myself in one of the early days of this blog. It goes into much more detail with arcane implements, with new wands, orbs, holy symbols, tomes, staffs, foci, and totems, for all sorts of character classes that can utilize such things. Up to this point, one could easily call Mordenkainen's Magnificent Emporium "Adventurer's Vault 3" (which it is) and be done with it. However that would completely belittle the fact that this book is organized and presented in a far superior way to either of the Adventurer's Vault books, and is ultimately a lot more useful than those books (when it comes to wondrous items, and other magic doodads). However, my favorite thing about Adventurer's Vault 2, immurements, did not make it back to MME, which was a bit of a disappointment.
The last part of the book concerns itself with artifacts and cursed items. It defines artifacts, broadly, as those sorts of items which are more important, narratively, than mechanically. Many of the examples they list are items created by or formerly owned by the gods, and which possess extremely potent innate abilities. However, the introductory piece on artifacts makes it clear that their proper place is as the "McGuffin," an otherworldly item which moves the plot in the adventure forward, and so therefore the PCs' possession of that item may be very brief. This is a concept which I think is pretty cool, and I think the only thing lacking is variety in examples. I think there were about two or three items per tier listed.
But the cursed items are exactly as you'd expect. Many are an obvious nod to earlier, more deadly editions of D&D, recreating some infamous items for the 4e world. One of the best comments that the book makes, however, is for DMs to consider: cursed items, by and large, are extremely powerful magic, requiring a lot of time and effort, and sometimes a lot of resources to bring together. Therefore cursed items should not be used lightly, or indiscriminately. Because of the considerations involved with the manufacture of cursed items, one must assume that most are made, specifically, for the torture and punishment of one specific individual. Cursed items for cursed items' sake, therefore, would be gratuitous. But this warning about considering the cursed item's past also is a very strong reminder to justify why you, as DM, are inserting this item into your adventure; what is its background, why is it there, who was it intended for, was it successful? The best part about cursed items is that they are virtually indistinguishable from the item that they are intended to copy until the curse is activated, in which case it is too late. Mercifully, though the cursed items usually cannot be removed after they've been triggered, it is not unnecessarily difficult to remove them outside of urgent situations.
Overall, a fan of older editions might criticize Mordenkainen's Magnificent Emporium's cursed items as not being nasty enough. But, overall, they fit well into the general schema of how things work within 4e. I believe it is by far the best in the "loot" series of books, but is also not remarkable when compared to the others. However, a completist will find it more than satisfactory, and if one were to only buy one of the three loot books, I would argue that the Emporium would be the one to choose. There has been a clear evolution of thought and style over the course of 4th edition's publishing history, and Mordenkainen's Magnificent Emporium reaps the benefits of this.