I had priest/lock to level along with my first holy priest (it was horrid leveling holy ages ago) and its quite awesome. Essentially just keep the priest idle except for pre-pull shield and lifetap maintenance.

The only problem I found was that the pets often ran oom too fast and manually casting taunts was the way to go. It also develops a lot of affinity skill. But with the priest you can quickly resummon a pet quite often.

When revisiting the warlocks and priest I really liked the new night elf changes to dump aggro.