I like Storm Guardians... But only for the special weapons, as the others have mentioned. I normally take 1 unit with Meltas (and my Autarch with Melta/Power Sword) to accompany my 2 squads of Fire Dragons, all in Wave Serpents. This gives me three mounted units with meltas, one of which having ObSec and a little combat ability. The Warlock is generally overpriced for what he does.
I played around with several large units (20) of Storm Guardians on foot, all led by Warlocks with Enhance and an Avatar. Unfortunately Guardians are too expensive for the horde approach to work and too fragile to make it across the table.