The latest Space Marine supplement, Angels of Death, is really, really cool. It collects some previously-released Formations and adds a few more. Notably, it has big Formation of Formations for each several 1st Founding Codex chapters. Combined with the Formations in the base Codex, I now feel like a Space Marine army can be organized around different playstyles, and do so in a way that is fluffy and characterful.
Ravenguard get lots of scouts, sneaky deployments and sudden assaults. White Scars get fast bikes. Iron Hands can field a tank-heavy force. Salamanders get some more flame stuff (yawn). My own Chapter's lineage, the Imperial Fists, receive a Formation that allows multiple devastator, centurion, and vindicator squads.
This is the book that is making me love Formations. My first reaction to Formations was pretty negative, but that was back when the core of army building was the CAD.
It now seems to me that GW is using Formations as a way to reflect the character of the armies. The new Formations of Formations are, in effect, new, customized (and customize-able) Force Organization charts. Not since the old 3.5 Chaos Dex, have I felt this much freedom to build a force the way I want and to have it reflected in the rules.
Can Formations be abused? Are they unevenly designed? Sure. But so is 40k as a whole. More than ever, it is imperative for players to meet and agree on the type of game they wish to play.
Here's hoping GW creates formations in this much depth for every army, not just Space Marines...