Rating this one is tough.
On the one hand, in this book, you have some of the very best monster design in 4e. WotC has only been getting better at monsters since Monster Manual 3 last year, and I haven't found much to complain about in this one yet... Except for one thing, which I'll get to.
Much like in the Shadowfell box set, we have a 128-page softcover book, a poster map, and six cardstock sheets of tokens. Unlike in the Shadowfell box, WotC doesn't even try to make the container box-like; it's a slipcover, shrink-wrapped, holding all the goodies. All told, this still makes the product rather pricey for the page-count... As I've said, this is quality material, so I'm not grouching too much, but it's a tough sell at full price. (Sadly, the tokens continue to have the monster names across the Bloodied sides, which make them easier to find, but make it much tougher to use them as generic placeholders, and make it nearly impossible to hide monsters' identities from your players. I'd rather guess!)
Also like in the Shadowfell box set, a large portion of the "monsters" herein are basically NPC groups, generally opponents. For me, this is awesome; it's very easy to take a group like, say, the Blackfang Gnolls or the Tigerclaw Barbarians, and quickly construct a level-appropriate encounter for my players. It helps make prep-time low, and play-time valuable. I'm also partial to NPC groups in general; a faction such as the Iron Circle or the Grey Company has a lot more interest as a long-term foe than a simple Troll does. This also makes it very easy for me to integrate these foes into my home Dark Sun game.
Although I've not had time to exhaustively read the flavor text, there's a ton of it, and it's overall well-written. The Wandering Tower, Mooncalves, and Dythan's Legion - the ones that caught my interest first - are all fantastic. The art is pretty uniformly high-quality, too.
So how nasty are the monsters? Calastryx was featured on the Wizards site within the last month, and he's the scariest 14th-level Solo I've ever seen. Google him or the Boggle for examples. Perytons are back in their heart-ripping glory, and Pennaggalans are every bit as gross and creepy as ever. I'd go on, but I'm afraid I'd just rave about nearly every monster.
So given this, why four stars instead of five?
Well, the monsters here range from Level 1 to Level 20; everything at 19 or 20 is an Elite or Solo, so this book is mostly useful up until about Level 17 or 18ish. There's not a single Epic monster to be found. Worse, somewhere around 3/4 of these are Heroic-tier; there's a sampling of 11-20's, but they're in the minority, and there are only a few per level. While this is fine in general, it overlaps with the Monster Vault significantly, and is of only moderate use in my high-heroic tier game.
While I applaud the new monster design, there's a dearth of Epic-tier foes using the upgraded design standards. I was hopeful that this book would cover all levels, but it doesn't. With this and with the high cost per page, I had to drop this otherwise outstanding product 1 star.
Ah well. I don't regret the purchase even a bit; it's just a missed opportunity.