I catch one in every 3 or 4 raids i do, which seems average.
I wait for the attack and time a curveball to hit directly after the attack, this way i only lose a ball when the legendary attacks 2 times in a row, which is rather rare. You cannot lose balls from the pokemon flying up or down this way.
So i got 3 Lugia and 2 Articuno. I got INCREDIBLY lucky with the IVs, i got a 96% articuno, a 93% Lugia and a 87% Lugia. 2 really good ones, one decent one, and two crap ones, really great for a sample size of 5.
Still want more Lugia. I will play every 4 star or 5 star raid for TMs and special candy, and to help other players, the raids i really chase are Lugia and Despotar.
Articuno seems strictly worse than Lapras. Lapras has lower CP, but is much bulkier, survives much longer, and seems to do the same damage.
Lugia just seems over-all great with the right moveset. I have no good allakhazam, but with two Espeon and two Lugia, i should be able to solo Machamp, i still need some good machamps because i also still lack some good attackers against Despotar.