I was torn between 4 and 5 stars for this, but in the end, I decided to round up because the price is so insane. For less than every other 4e book to-date, you get...
* A *hardbound* book with about a hundred pages of tough, scaly, chitinous, psionic, spiky, mutated beasts perfect for Dark Sun. (According to WotC, it was supposed to be softcover, but the printer's error is our gain!)
* A 20-page section on Important Personages, including all the Sorcerer-Kings and a few other notables from Heroic to Paragon levels. (This section also has stats for some templars for several of the cities.)
* A very nice, very important section for adapting existing monsters to Dark Sun. Monster Themes are included - many of which include trade-offs, where the monster loses one thing to gain another.
* A bevy of hazards for travel through the Wastes, fighting in the Arena, etc. The latter two sections take up the remaining 20 pages.
Overall, it's impressive as heck - especially for the price.
Naturally, it's not perfect, and for a few minutes, I was downright peeved. I couldn't find my favorite Dark Sun monsters - the Dwarven Banshees, Elf Dune Runners, and Athasian Sloths. Well, the Sloths still aren't here (doh!), but folks looking for those brilliant undead can check the Wight section. No idea why they're there, but they're still very true to the originals.
I also wish that more of the Sorcerer-Kings had had MM3-style tricks to throw off conditions. At high levels, I can see the Dragon of Tyr getting stun-locked, and that just makes me sad. Some of the Sorcerer-Kings have these sorts of tricks and some don't. Also, oddly, some are Solos and some are Elites. Those with a bent towards simulation might get distracted by this, but I think it makes sense - the Elites are just more likely to have fully-capable retinues. And, as one more tiny gripe in the "just to annoy us Dark Sun oldtimers" category, some monster names have been simplified (b'rogh -> brogh, hej-kin -> hejkin), and the plurals have all been normalized.
Overall, I'm thrilled with this purchase. I would have readily paid a few more bucks for this same content. It's a must-have for anyone running Dark Sun...
...but if you're not running Dark Sun, it's likely not be worth it. About a third of the book will be mostly-useless to you, and a lot of the monsters might feel incongruous in Forgotten Realms or the Nentir Vale. You could always reskin the cooler stuff, but you'd likely be better off just using the DDI Monster Builder. Still, for the price, it could be tough to resist.