Manticores are 6 RHD magical beasts, best known for their ability to shoot tail-spikes. Otherwise, they're just another boring bag of HP with wings.
At first glance, their tail spikes' are a wonderful source of bonus damage-stacking, but apparently they're limited to twenty-four spikes per day. At six spikes per turn, any manticore is going to run out quickly, at which point their damage output drops significantly. Unless you rarely fight more than one combat a day, investing in the spikes just won't pay off.
Ignoring the spikes for now, let's compare manticores to the griffon, another low/mid-level magical beast. The manticore has somewhat higher ability scores, better natural attacks and one less HD, the griffon has superior flight, pounce, and rake. With the griffon getting +0 LA, I feel like a similar score could be justified for the manticore.
+0 LA assigned: next are medusas.