Apparently devils, the schemers and corruptors of ancient legend, now look like evil glowing lions. How the mighty have fallen.
Hellcats are... not so great. They lack many of the devils' racial traits and have no spell-like abilities (or summoning ability) at all. They lack reach despite being Large and can't wield weapons without giving up their strongest natural attack.
They're not completely horrible, though. The ability scores are quite good, and pounce is something all melee builds need. Rake attacks are icing.
Finally, there's Invisibility in Light, a strange ability. In light it makes you invisible, in shadows or darkness you can be seen, then in magical darkness you're invisible again. I suppose extraordinary permanent invisibility is worth something, but countering it won't be hard.
All in all, hellcats are a decent option for someone who wants to play a big monster with a neat trick. Are they worth +1 LA, though? Probably not.
Current LA is +0.