The way I handle a fight with stirges was make them multiheaded [cryo] stirges. They swooped down on the party, breathing cold on them first. Then, they moved to attack, attaching themselves to the players. With two heads, they were gauranteed to suck 4 con from the player and with about 30 hit points, they weren't easy to kill. One player took 12 Con damage from 3 stirges. Man, my group will never look at stirges the same way. Oh yeah, they didn't like the Multiheaded [pyro] half-troll Stirge of Legend either. Hehe. That was a fun battle...
With normal stirges, I think you need a large swarm, probably at least 2 per party member because once they attach to you, their AC drops considerably and their hit points are low. They also lose their dex to AC and are sneak attackable. There needs to be enough time for them to survive a round to drain to become a real threat and by threat, to be challenging.
The flight and the size let them come from anywhere. Think about the group in an abandoned house. They are checking it out and they hear a buzzing. The group looks around but sees nothing. Then suddenly, these tiny little creatures come from everywhere, cracks in the floor, ceiling, walls, inside the chimney, etc. I think it would be more frightening for them to be all around you than to see them flying towards you from a single direction. (Unless they're mutated. Then you want the group to see them coming and wonder what the heck is up with the stirge.)
Aw well, good luck on the Battle of the Stirges.