I have to admit when I first read into the extra detachments I was a little worried that it would be too powerful and prevent players using the combined arms detachments to make an interesting list. I will however day that the positive thing that stood out for me was that GW are making new ways to play the game instead of Unbound and Combined Arms. Any alternative way to play the game is great, it stops it becoming stale and allows variance in lists. Obviously for this to happen each detachment in a codex should be internally balanced so to allow for plenty of variety in lists on tables. Along with this it would be fair to say that the "Objective Secured" rule for armies using Combined Arms cannot be too hideously out gunned by whatever the in codex detachments can offer as to allow fully customisable lists, without going down the crazy unbound route.
A little more research got me to the point of noticing that what you really want is the "Detachment of Detachments" one. This is more flexible than just using one and provided there are multiple ways of fielding this it can at least make numerous lists, so it's not all the same "Be all, end all" list every time. I do however think that the danger of this is seeing too many of those same models over and over. As I write this blog post I should probably point out my focus is a non-competitive one, I'm really only looking at what's going to be played in games with friends or club members etc. Obviously some detachments are crazy good for tournaments but that's not really my thing.
So in this casual mindset, my verdict on detachments is that they are good, not great but good. They give you another way to play the game and that's awesome. Tie this in with scenarios and such and you could play a lot of games before playing "the same game twice", which as a casual gamer is important to me. Any maelstrom of war game really provides something interesting for the two players to face, forego "Objective Secured" ,which definitely makes a difference here, or go for something with stronger "special abilities". It's a real dilemma faced by gamers that I have been against and talked to. Obviously however when you go kill points or maybe eternal war missions there is much less debating and unbound/special detachment lists will be more common. So yeah, I like them from a non-competitive point of view and hope that I will have some models to try one out in the near future.