I did 4 warlocks 1 priest all the way up to 70. I was able to complete every instance I put my mind to.

For that demon elite in the pool of aggonar in hellfire I had to get some help :S Had I waited another level or two I'm sure I could have done it, but I wanted the trinket!

My warlocks were all specced demonology (51 points down) and I'd run with 1 imp and 3 felguards. I'd go into demon form and use immolation aura when things got hairy. It was pretty fun!