Yesterday I was messing around in a dolt game with the devastator to AWESOME effect.
Turns out, the bolt pistol is awesome. I messed around a lot, and if you use the super stomp or fast stomp, as well as another armour boosting or based skill (such as the AOE damage when it deactivates, or the iron halo/chaos icon) you can turn into an absolute melee mountain.
First things first - you run faster with a bolt pistol. In seize ground, this is awesome. Run to the middle and deploy with your heavy bolter, and follow it up with some LOLOLOLOLing. This is probably the thing I see the least - nobody is weapon switching at the moment, and it means devastators are slow to get out into the field. Every second counts, and the bolt pistol will save you a number of them over the game if your running to and from locations.
Second, the bolt pistol itself - when rapid fired at close range, it has a potentially higher damage output than the other weapons, plus you are able to strafe and move easier. This makes it ideal for dealing with assault marines or ballsy tactical marines, and even other devestators who are terrible at aiming.
Finally, perks. The BP perk itself is limited to assault marines, which means you can't take it, but that doesn't make the weapon any less powerful - combined with the stomps (which push back and severly damage folks) you can shoot, pop a stomp, then quickly headshot or fire a few more off to finish them. Still not enough? BOOT TO THE FACE.
I used this build last night. Pissed a lot of people off, and I was surprised nobody else was using it.
P.S. Pocktio stole this build from me.