I still use my Storm Guardians, if only for nostalgia, and I rather like the models actually (the pony tail helmets specifically). They also make for some good variation as apposed to taking more avengers. I find myself using serpent bound Defender Guardians more now.
They aren't as good anymore since Warlocks aren't guaranteed to get destructor (I like to use them as a flamer squad). That alone would have been fine, but not being able to assault the turn the disembark really hurts. Even though they aren't really good in assault, they hit before most opponents, and could mop up fairly well after 2-3 flamer attacks. It's just not even an option anymore.