The Flails were my most used weapon on my Warrior playthrough of NGS2 and it is so far on my normal mode playthrough of NGS1. In NGS1, they have good invincible frames, particularly with X,X>XXXXXX (er, replace "X" with Square.) Pair it with the armlet of potency and all those kicks (like f+triangle/Y and X,YY) do serious damage, even to bosses.
They don't have as many i-frames and they aren't great for bosses in NGS2. Weapons like the scythe, enma's fang and lunar are great boss weapons. but that's mostly because the bosses in NGS2 are simple as hell, aside from the second Elizabeta fight, imo. They don't require a lot of precision.
The flails are fun in normal fights since they de-limb easily and hit guys in a 360 circumference...this is great with NGS2's mobbing AI. And once you get it powered up, those fucking insane combos where you spin the air are amazing. X,X, f+XXXXX,YYYYYYY is awesome.
<3 me some vigoorian flails