I just did the mask quest today.

Started out alone, then a guildmate joined (both lvl 30), and over the course of the afternoon we had a full group of faction hungry people going.

Respawn in the mine is 10 minutes. We usually had to wait about 30 seconds after clearing it fully.
Getting the faction was no problem at all; though the 4 nameds for the final step were not showing as much as we'd have liked to.

I started around noon, and finished at 22:00. Yes that's right, 10 hours later I finally got my mask.

Worth it? Yeah, I like illusions 
Smart? Probably not. Still a 30 hp upgrade over my old mask as well.
As has been mentioned, it won't stack with our runspeed buff, horse, or whatever, but there's just something about being a panther.