The Monster Vault is such a great idea it makes you wonder why nobody ever came up with something like it before ^^
Like many DMs, I don't like the idea of having tons of great monster stats but nothing to adequately represent them on the table in a minis-based game like this one. The Monster Vault really deals with this problem in a cheap but effective way. Of course, the tokens in their 2D simplicity can never have the same visceral impact a well-painted miniature has, but they tell the players enough about the menaces they face that the encounter comes to life in their collective imaginations.
So, in short, I think the tokens are great. But that's not all. The new pocket-size monster manual is rich in content, features revised stats for all the monsters in the original MM (and more) and it also features more lore than the original book. Of course, something has to be sacrificed for this additional content and in this case it's the Knowledge checks. The original Monster Manual contained information the DM could tell a player based on the results of their Knowledge Checks. While that was a handy feature, I don't think it's that important and it isn't all that missed in this manual.
I don't use any other Essentials products (not thus far at least) and I don't see any compatibility issues with the core 4th Edition stuff, so that's not a worry either.
Besides the wealth of tokens and the juicy book, there is also a booklet with a 4th level-adventure and additional battlemaps. I've only glanced at the adventure as I prefer to write my own for D&D, but it looks like a fun romp, only hindered by the fact that it (like many D&D adventures of late) is extremely combat-oriented. I deplore this a little bit as it reinforces the stereotypical misconceptions many people have about 4th edition ("it's just a tactical minis game, not an RPG" they say). I disagree with this assessment as I have had some awesome roleplaying experiences with 4th Edition, both as a DM and as a player. It's as story, investigation, combat, puzzle or whatever-driven as you want it to be, in my opinion, and features the necessary rules for any kind of adventure. Still, Wizards of the Coast seems to insist on filling their offical adventures with lots of combat encounters and not too much characterization and investigation (something that Warhammer Roleplay does much, much better in their official publications IMO).
The battlemaps offered are cool - just the fact that they're there is nice. It's just one double-sided sheet, however, and while one side contains a beautifully rendered encampment, the other just consists of a dungeon made from the separately available dungeon tiles. Also note that while the included adventure does feature both sides of this map, they are not sufficient and you'll need either the dungeon tile boxes or an erasable mat of your own to run this adventure in full.
All in all, this is a great product featuring noteworthy innovations. It's still not entirely "everything you need in one box" if you're looking for that, but the thinking behind it shows a definite move forward for the whole of D&D in my opinion. Also, for this amount of content, the kit is really cheap, especially compared to similar expansions for other games.