Are they good for putting on mass?

They're good for keeping the amount of time that you're in the gym down.

I switched to supersets for a while and it cut my gym time in half.

are they that good for mass?

They won't really add or limit your ability to add on mass.

I can't say if it does or does not help with mass (but i'd prob listen to ev), it does however blast the hell outa your muscles.

I do suppose in fairness, studies have shown that if you were structure your super sets in antagonist pairs (chest/back, quads/hams) that you'd have a potential to lift more.