I used my 70 lock to boost my shaman to lvl 40 or so. It was really easy. I could have easily go to 60 or so i bet. I was felguard spec and I just hit everyone with curse of agony and set my felgaurd to defend. He destoryed everything with the cleaves he does.

The only 2 things to remember:

1: you lock has to agro not the pet - otherwise your alts dont get XP from the kill. I did severl runs before i realized this.
2: use demon armor - the armor bonus goes a lot farther than the +dmg bouns. I found that I went faster when I did less dmg but took less damage and didnt need to heal as much.

GL