Guardians are the best siege units in the game. They have a range longer than defensive anti-air structures. They also can deal a great amount of damage in a very short time. However they are expensive, and are slow units.
Massing guardians with mutalisks used to be a common attack, but now there are too many counters in Brood Wars. First goliaths and valkyries will shread mutalisks/guardians. Also corsairs are effective against mutalisks, killing the escorts to the guardians.
Try to stay away from using guardians, because they are very expensive. It doesn't take much to kill them. Use queens to break into well defended bases. Guardians aren't really the answer, unless you totally dominate your opponent. They are good units to defend against marines/hydralisks/dragoons.
Whenever using guardians to siege a base, use your terrain to your advantage. Guardians fly, so attack from above cliffs or rivers. Also air units tend to bunch up, so guardians should be spread out. This will ensure that you won't lose your 6 guardians to a single templar.
