I play demonologist and there are two or three types of demos...
Havoc/General specc....powerfull for his range ( 32 meters ) and great dmg he has 2 roots one knockback ( can knock down cloth, light, medium and heavy ) one instant stun and one 0,5s casting time stun, your pet can heal you....very good in mini, BUT when you are close to him he is quite weak because his power is his great distance. When you use pacts they deal you dmg so its very dangerous. Otherwise very good demo most seen in the game right now.
Conflag/General specc....I think this demo is weakest from all three types, BUT he has enourmous aoe dmg and he can knockback down more players with detonation even full plate when you have stacked 5times debuff for using it. ALSO fires of gehena is 2s casting time so its faster than shockstrikes and THIS demo can use PACTS without any dmg deal to him from pacts and he has let them burn best aoe spell in the game with 40s cooldown, so he is very good in minigames when you have skilled group.
Hybrid specc....For me the most powerfull demo even with cc nerf. You dont have a purge (antiroot) but its quite useless when you have immunity only for 3s and then they can root you again...then you have one knockback against cloth-medium and one against all types of armor also you have 2 instant stuns and ofc two roots. You are flexible so you can use gehenas for short range and shorter casting time and when enemies are far away you can use shockstrikes. Also you have healing pet. Weakness of this demo is you can be run out of mana faster then with one tree/general feats you dont have purge so you have to survive roots and you dont have feated +armor and +protection also you will lost +70 magic dmg from general tree. But with expansion where you have in aa spells with fire and electric dmg I think this demo will be more powerfull.
I played conflag/general but nowadays I play hybrid specc for long time and I wont change it its very good. But its your choice and choose what you would like to play. I think these are all types of demos.