Yes and yes.

It used to be that it did stack with everything if you put it on then chunked across a zoneline as you would keep the buff and you would lose the visual of the actual panther form. Now it doesn't do anything at all. Sony fixed it and while I haven't messed with it personally myself, the last patch apparently caused it to do nothing but make you a panther..no more run speed.

I think you can sum it up like this

If you want to be able to turn into a panther than get it.  If not don't. Aside from the look we really dont need to get it as the Aum doesn't stack with it or the horse.

Aside from a handy lev the same goes true with getting bat form too. (You cant even soaring leap in bat form  That would have been cool though if you could.)

~QTM

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.

It does not matter that it does not stack with our run buff. I use it when I have no outside forms of runspeed buffs available. Such as in a dungeon crawl where I want to have my damage buff up and have no one in group that can buff runspeed.