I think she's really good. Two swooces, range, great zoning and options, fantastic grab game. I keep her as a secondary since she covers certain matchups really well. Really slept on, though, and some other top players I've talked to put her down lower on account of her slam.
Luna is the queen of safety. She's small and has the best mobility in the game, aside from Dronkey. Combine that with zone control using her AB bats, projectile, and sneeze and she becomes almost impossible to hit. There's your advantage in most matchups. Her only problem is that she lacks damage output (her best combos only do about half of a slam meter), but so do Puss and Dronkey who are still top tier.