Formations in 40k

Cast of Players in progress
Formations have been around since Apoc v1 (released 2007).  However, it is only really in the past year that formations have entered into the core game of 40k and been getting smaller.  While large Apoc formations are still there (see the Grey Knights single formation), it is far more interesting to me to consider the impact of medium (<1,000 pts) and small formations (<500 pts), like Cast of Players for the new Harelquins.  These allow someone to complement an existing army with a different set of models and potentially bring something very different from their main army to the table-top.

I am very excited about including Cast of Players in my Eldar army.  It has driven me to consider formations more broadly.

CAD & Allies Detachments
When 7th was released in spring of 2014, the internet was heralding the death of the Force Org Chart (FOC) and how broken Unbounded armies have become.  But I think the curve ball in all of this has been the introduction of alternative FOC and Formations.  From a gaming perspective, Unbounded armies represent the "entry" point of learning the game - count up the points of the models you have and play.  Next is the Combined Arms Detachment (CAD)/Allies Detachment - these are FOC for bringing your tailored solution set - downside being the mandatory units you have to field.  My Ultramarines fill out at more than one CAD, maybe two, minus the Lord of War and Fortifications.  The Eldar are built on a CAD for the most part.

Small and medium formations and army specific detachments I think transform list and army building.  The army specific detachments enforce a level of fluff onto armies, while the formations go even further.   Formations, especially those with varying special rules, impact army design.  Some units that are sub-par in a CAD become good options given a formation special rule.   These special rules can be unit, if not army, defining allowing the formation to accomplish table-top roles they otherwise could not perform.

On the hobby side, smaller formations allows hobbyist to build out a complete somethings without having to invest in a full army.  The willingness to invest less than a $200 in a playable combination of units (to include the book) is nice given the current costs of buying a complete 2,000 pt army. These are also much faster to paint, again giving that sense of accomplishment sooner, rather than always distant.