Firstly, I run Peter with a very generic role in mind and his bio-morphs reflect this. He always hangs out with a unit of gaunts (usually Devourers) to provide some proper close-combat support to my weaker models and some extra light firepower. Equipped with dual boneswords, adrenal glands and a deathspitter he is a little over 100 points but does his job very well. The deathspitter can deal reliable damage but it is in the assault he really shines: charging with the gaunts he strikes first (I6!) so his power-weapon cannot be diluted through wound allocation and at a whopping S6 his numerous attacks usually make a decent mess of whatever he meets. This is great for providing a little surprising punch to a gaunt unit to help them definitely kill that unit in the assault. Even after the initial charge he is very effective which helps swing combat results when his gaunt bodyguard get only half the attacks in subsequent phases.
Another great use for him I have found is assassination. With my big Swarmlord and Tervigons hanging about little old Peter slips through the net most times unnoticed. This means he can go and make a mess of an enemy commander, special unit or similar which is made much easier with his dual boneswords (remember the 3D6 instant-death!). I once took down a Demon Prince who otherwise would have wreaked my army with Peter.
Finally, as he is such a small target he is often ignored and when put in a unit of gaunts the whole unit is lost among the monstrous creatures and other targets. This lets Peter become a very firm rock of synapse when my Tervigons start dying and the Swarmlord is away from the objectives beating face. Being 'babysitter' may not sound like a great position but it is often the crux of a game, keeping synapse on an objective and keeping the Hive Guard picking the correct targets.
Overall, my Tyranid Prime is my Swiss Army knife, able to fill any role I want him to from close-combat assassin to fire support. I think he will remain in my standard army for the time being.....