I don't know about felguard, but I guess warlock pets would be about similar to hunter pets, so I will share my experience.

I ran four hunters and a druid healer from 1 to 80, and they were a very fun a viable group. I cleared all instances on my way and then converted the group to PVP.

Pets do fine on quest elite bosses and normal instances. As you guessed, heroics are not really an option, though.