i love my sentinels!
i think the trick to using them well is to realise that they need to be hidden from enemy fire as they do tend to fall down if the enemy so much as fa.rts in their direction. get sneaky with them, cover hop them till they can get a side shot on armour and give 'em some infantry backup. just don't have them roaming the open fields between cover as they will get dead rather quickly.
on the subject of hoard control, 3 catachan sentinels with flamers and hardened fighters can really slow down a big mob of gaunts and allow the infantry to counter charg. just a thought.