The Warrior Priest and Disiciple of Khaine are mirror images. They both rock in 1v1 and can do very well in PvE soloing. Admittly pre-20 your healing abilites are lack luster compared to other healers. Once you hit 20 you git your first AoE healing spell and that is were you start to shine. WP's/DoK's are awesome group healers for melee classes. On top of their healing ability they must also be in melee combat to regen their fervor/essence in order to heal other players.
Essentially you go into combat and heal...You then have to build up your fervor/essence to be able to cast your heals again. You can only build up fervor/essence by using combat skills. Now the thing about WAR is that there is no mana bar. Instead there is a action points bar which starts at full and each time you use a combat skill it goes down a few points. So as a WP/DoK you must juggle healing with your combat skills making sure you have enough AP to use your combat skills in order to rebuild up your fervor/essence to heal again. AP regens at a fairly fast enough rate so that you can heal and then switch back to combat as long as you don't spam your combat skills to fast. IMHO it's not a class for people who like to sit back and heal or who want to only press one button while they play. In other words it's a very involved class that takes use of almost all your skills and concentration especially in PvP. Again level 20 is where you start to rock because you can esentialy AoE an entire group and still fall back on your 2 single cast HoT spells to top people off.