I get what mutilators are about, in principle. But: (a) I don't like the miniatures; (b) they might have been better wrapped up with obliterators, rules-wise. Those are just personal opinions.
Mutilators can be bought in units of up to three models, and can obtain any mark, as well as Veterans of the Long War. All of these options are very viable. The Marks of Khorne and Slaanesh and Veterans will improve the close combat effectiveness in all probability, whilst the Marks of Tzeentch and Nurgle will improve survivability. There isn't a bad upgrade option here to be honest!
But here's some issues. They deep-strike, but can't assault on the turn of entry. The can go in a land raider, but there's better units begging to go in land raiders that are not only more effective, but also fill up the transport availability better. They might have 2 wounds each, but they die to instant death like any other marine. A lucky vindicator will deal with these guys in a single shot.
To illustrate these points, consider this set-up:
3 Mutilators, Mark of Khorne, Veterans of the Long War (192 points)
They're getting a similar number of attacks on the charge (only one more in fact, unless we're talking about a pair of lightning claws) as terminators with comparable upgrades, but there are less of them. Plus the weapons that they're going to manifest are limited such that they can't have the same ones twice in consecutive player turns. Terminators can and do!
On top of all that, they're competing for elites slots with Terminators, Helbrutes, Cult Squads, and others who may be better purchases. So, I can't shake that negative feeling I have about these mutilators, sorry.