I like the guardians, but according to eldar fluff, I have a hard time using them. If the race is on the path to extinction, I wouldn't send partly trained, weak armored civilians into battle. They are good for sticking to your objective in the backfield, but usually get hosed down pretty quickly when on the front line. I usually use large squads of them, and hope the other player has better targets to shoot at.
One time they did end up driving off and running down a squad of genestealers, much to the surprise of the hive mind controller! (granted the genestealers all attacked some harlequins who were in the melee, but the guardians did win the battle and ran them down.)