From our esteemed moderator and industry professional, Failure2Stop

F2S taught CQB and small arms to the Brits and had the opportunity to shoot this; actually use it for years. He was sent to the Royal Marines in between combat rotations with the US Marine Corps.

An excerpt:

The lack of adjustability of the LOP (length of pull) makes the gun sub-optimal for CQB. Everybody touts the thing for being so short, but the LOP is barely shorter than an M16A2. Combined with the zero amount of eye-relief of the SUSAT; CQB work with it when wearing armor sucks unless you want to rely solely on the laser (if you get one) or until the ACOGS come in (which have a MRD piggy-backed). The long LOP prevents the 3-man from carrying in the high port, which results in a less than speedy 3-man's gun in the room/fight.
The rest of his discourse is here.