To be honest, both Qyzen and Tharan are very good compagnions. If I wanne go melee and dps myself i choice tharan, if i wanne do more cc and healing i take qyzen. They are both fun the have as a comp
I've found Qyzen to be hit or miss depending on which way you go as Consular.
I've loved him on my Sage, just send him in to draw aggro and keep him healed while blasting away from distance. I faceroll mobs with him on my Sage and hardly have any down time between fights.
Can't stand him with my Shadow, he dies very fast, no matter how good his gear is and he is an awful companion for Shadow. Have to meditate after every fight and any fight with more than one strong mob is a toss-up. Theran is like mini-god mode for Shadow once you get him and I haven't used another companion since. Sure there is a bit of drop off in DPS, but I love that I never drop below 80% health in an elite fight and I can solo Champions with Theran's heals. Not to mention the extra interupt is nice.