I was sceptical at first, but reading this fun sourcebook convinced me within minutes, that I've made a good decision in buying this. Assassins and Necromancers have always been my favorite fantasy archetypes and I'm happy to see them transferred to 4th edition without any annoying 3rd-edition-fallbacks or uninspired and lackluster mechanics design.
Although the Deathwarden and Spiritsworn seem a little redundant against the shaman or avenger to me, they're still nicely done and have a unique flavour to them.
Assassin and Necromancer are my favorite parts of the book and I can't find anything bad to say about them. Their powers are fun to read and seem to be balanced enough (let's see if WotC can keep that balancing up). Even better, they feel unique. The assassin is not just a rogue with more poison and deadlier backstab, and the necromancer cannot be mistaken for a dark wizard character of some sort. All four classes are really unique in style and mechanic. I like it. A lot.
If you long to play an Assassin or Necromancer in 4th edition, this is IMHO the only viable solution so far.
[5 of 5 Stars!]