I have different items with mana syphon and it works with all - weapons, armour, and charms. But as previously stated, you must be fighting a caster mob for it to work at all. Caster mobs spawn 1/3 of the time (or less) and if you have under 10% MS then it's likely that you won't see anything except every once and a while.
My level 45 heavy caster setup has 23% total mana syphon between my armour and charm and I definitely notice a difference