At 198 points, a squad of 10 Rets with 4MM isn't "cheap". But it doesn't have a degrading profile, it can take advantage of cover readily (as mentioned above), and with an act of faith, it can be quite the sledgehammer. The most important thing about them is to range your opponents right. Some careful measuring can get them just in range of what you want (24" is a ways, especially compared with melta guns or inferno pistols), and they are easy to screen with our tanks or terrain. I've had a lot of fun pieing the corner on just the tank or tough target I want to kill, eliminating it, and leaving a frustrated opponent with few return fire options.
I used to run all-mech in 7th, but since I don't field repressors, I am always on the lookout for ways to get the most out of sisters on foot. This edition is much "faster" than 7th as far as speed. Guardsmen can readily outrun tanks with the right orders; retributors with MM can reposition 12" (move + AoF) and still hit on 4s, making that 24" range deceptively larger than you think it is! And many opponents outshine sisters in CC and will bring the fight to you. Fliers, too, often err dangerously close, and the MM punishes them for that.
Now, if I did field repressors, I would absolutely make a MM bunker, plop it in the middle of the field, and defy the enemy to come in reach. As it is, I'm thinking of trying a bastion in my next list just for the hilarious giant zone of denial. I'll let you know how that goes.
One thing I will say about retributors - they really do want that AoF support. The sudden unexpected move with them, or better yet the blistering double-tap, is what really makes them shine. Chaos gets a 2CP stratagem to shoot its slaanesh havocs a second time; we can do it turn after turn without spending a single CP. Stacking our special weapons into squads maximizes that efficiency. The range and power of the MM really brings that out. Since I run brigades, and have to fill the elite slots anyway, I don't mind taking 3 imagifiers (or 2 and a hospitaller if I am pressed for points), which gives me 3 reliable AoFs per turn. Usually they go to the 2 seraphim squads and Celestine, but it turns out seraphim thrown at the enemy face die quickly - especially if they swoop in with melta pistols and slag something important - and the rets are a great target for it on t2 and later.
I've gotten good use out of HB rets as well, usually run min and also with an imagifier and a canoness nearby for support, but since I only have 6 HB sister models and I tend to run them in my brigade BSS... I use this unit less often. I haven't given HF rets a shot yet.
The exoricst... to be honest, I field it mostly because a sisters army can be quite monochrome, and the exorcist adds a splash of variety. Otherwise, sisters are sisters are sisters, whether you call them battle sisters or dominions or retributors... they all look the same. :/ I like having something else on the table. I don't really take it because it is terribly useful, though. Not useless, just underwhelming. Yes, the T8 is tough, and yes, it has a 12" reposition (but so do rets...!), but it rarely gets touched, and its damage is... well, not quite wet noodle, but it leaves a lot to be desired. I'm hoping it gets buffed to a basilisk-style 2d6 take highest, or gets some kind of strategem support when our codex does drop. I do enjoy two things about it: first, it is great at "clean up" - it has the most firing options of any sisters unit and so I always save it for last, to plink off a few more wounds on whatever my melta hasn't ripped apart, or to take a flier or large vehicle down a notch on its stat block; and second, because it is AP-4, it is very satisfying against stuff without invulns. Since I also play chaos and field quite a few lascannons, the difference between ap-3 and ap-4 is felt very strongly. It's nice to land a hit and realize that the opponent just doesn't get a save...!
If I had 2 of them, though, I wouldn't field the second. I often times have to 'make' myself field the first. I prefer filling heavy support slots with penitent engines... but that is a topic for another day
Edited by Mekhitar, 17 April 2018 - 11:24 PM.