Dude!! I <3 my blueberry!! they are one of the best companions you can have especially for PvE! I'm demo specced so my boy give me 10% less damage(master demonologist), absorbs 20%(soul link) and give me spell power equal to 15% of his total intellect and stamina combined (Demonic knowledge)!!! At 48 that's just about 90 extra spell damage with nothing else i think(could be wrong). Then all you do is put 3 points in demonic resilience so he take 15%less of all damage! after that he becomes a super tank, and i gotta say my demon names are pretty awesome:
Imp- Pipkin!LOTR much!?
Blueberry- Kontast! Awesome!
Succy- Anglia! makes her seem even more like a stripper
FelHunter- dont quite know this but it's close to Ghrzzaman

And plus, to help shed some light on this matter, each pet has its own strengths and it all depeneds on how you play your lock that determines what pet is the best! I have my succy out for farming Soul Shards and for PvP because, since i'm demo spec, she give super damage abilities at 48, and i have the felhunter for spell caster because of the resistances and his spell lock, and i use the blueberry for instances and mob grinding and the imp for instances or party things for the stamina and extra quick damage! So if you're an aspiring lock, then how ever you play determines which demon you should use and if you need help, ask around the city or whenever you find another lock, most of us are really nice people and will have no problems helping out a little!

~Kaskel 48 demo lock
Scarlet Crusade, Reckoning