The sentinals do antiverylight damage -- they can't do more than dent cobalts and teh like, but yes they can shred fighters, bombers, and light armored craft like LRMs and siege frigates.

Thats right. There's a chart floating around somewhere that breaks down the damage modifiers in detail. In fact, I think it might be stickied in this forum. Seeing how a sentinel does antiverylight damage, it will be effective against fighters, bombers, siege frigates, and to a somewhat lesser degree, long range frigates. Against ships like Cobalts and larger ships, it only does 25% normal damage against those armor types. So roughly, only 6 points against most ships with heavier armor than LRMs.

The sentinel will be somewhat more effective against LRMs than flak frigates or defense vessels because of the phase missile's shield penetration. But then again, the ship will cost more too, so it balances out.