Hawks really, really, REALLY eat through Light Infantry . Guard, Nids, Orks, DE, and other Eldar are somewhat susceptible to Hawk Fire + Grenade Pack, even without Doom.
Hawks really start to shine when Doom is put on an opposing unit. Coupled with an Exarch's Sunrifle, and even Marines start to fall to Hawk firepower (weak but plentiful), and if its Orks or Nids they just eat it.. They also have Fleet, which allows them to move 12", d6" run and still charge. Not as useful against infantry (due to low STR), but with Haywire Grenades and Intercept, the Hawks can put an end to any vehicle quickly.
My favorite tactic with Swooping Hawks "ok, Exarch has Sunrifle, thats 6 shots." Opponent: "WTF, 6 shots".