Got LoM in the mail this morning, and I must say, I'm very impressed. While the book doesn't reach the high standard set by the Draconomicon, it (at least) equals Libris Mortis (which, by my standards, was quite good).
The Chapters detailing the various aberration types are extensive, but to the point, and provide nice flavor. IMO, they could have cut down a little on the various "additional" monsters, because ... well, after having seen the nth variant of "tentacled, slimy worm-thing", it gets somewhat old. OTOH, almost all of the descriptions fill a certain niche and scream for using them in an adventure. The PrCs are nice in that they provide some anti-aberration flavor for each class. I was less impressed by the aberrant feats, though. Some of them just scream broken (the aberrant wildshape thingy), while others are somewhat boring.
WotC is doing a great job as of late, concerning the modularity of their books. The inclusion of Psionics content as sidebars and alternate statblocks is just what I need.
The art, overall, is great. There are some ugly pieces in there (guess who's responsible: yep, Crabapple's back), but to make up for that, the WAR and Wayne England art is nothing short of stunning. My favorite pieces were the alhoon (by WAR) and the illustration on p.123 by Michael Phillippi.
There's some botches in the editing (again), though. One should think, that using the search function once to filter out the "page XX" references would be possible, but apparently it's not. Still, nothing that detracts overmuch from enjoying this book. I'm looking forward to John Cooper's review of the statblocks, though

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Although there are a lot of minor issues with this book, I think it is a great resource for DMs (less for players) looking to run an aberration-based or planar campaign. Four out of five stars, or something like that.
Oh, and on a side note ... diaglo must have been involved in writing the chapter on aboleths, because on p. 15, it is stated that:
Lords of Madness said:
All else in the world is a pale imitation of their primal perfection.
