It's great as an edgeguard or if somebody's in lag. However most people will just just shield it or sidestep and proceed to grab or fsmash.
The knockback is great yea, but hitting people with it is another issue. I can't remember the last time I managed to RAR bair a competent person that wasn't a Bowser/Charizard/DDD on the stage.
I get an sh bair off sometimes, but more often that not I get punished for trying to do it.
It's not entirely useless, but it's not that great. It has potential though and I'm sure we'll eventually figure out some way for it to be viable at time passes by, but right now I don't really care for using it that much. On the stage at least. Her bair is one of the things that is really not helped by her Brawl size at all.