They can be used, but are rarely any effective (so far they have been once for me and that's because my opponent made a big mistake). Other than that they have always been a huge disappointment, even with a Warlock backing them up, or another melee unit nearby to protect them, or cover, they still go down like flies to just about everything. Definitely not worth their points cost and they are not a viable heavy weapons platform either. This being said, there can be times when they are useful, but those times are rare and even then Avengers/Storm Guardians would probably have outperformed the regular Guardians.