There will be some mobs that you will do amazing on and some you will suffer (time wise) on with the priest / warlock.

There are some nice areas depending on your server where you can really dig in and gain xp quick. The epl/slime cave and the non-instance DM/ogres.

Setting your hearth to outlands at a early level via a warlock is a good deal, that will make travel to all the cities quick.

Getting a monster shard bag and the ST trinket asap is good too.