Best use IMO is on a class that doesn't normally come with familiars. Some of the magic abilities are pretty nice (free cantrips/spell slots/focus regen specifically), and this means you don't need to multiclass to grab them. The mobility and sensory options are decent enough but you have to go out of mask form to use them. I guess the mask stuff has some intrigue use too (smuggling the pet in, maybe a disguise, etc) but won't be super useful outside of those contexts. I particularly like it for clerics, as there are some nice domain powers that free focus regen would be nice to add in, though Champions Bards and Monks could use them too. And a cleric whose familiar is a mask of their deity's sacred animal might be neat.