This is a great addition to the game. As a cleric, I was worried that mercs might sort of "displace" me and my fellow priests. However, I think most people that one would enjoy grouping with in the first place are going to prefer a "real" cleric over a merc. Not everyone, of course, but I think it would be the popular choice.
As far as tank mercs are concerned, they make good meat shields (not the best, but will do in a pinch). My druid employs a Halfling tank for when she doesn't feel like kiting and my cleric has hired a tank merc for added DPS when soloing, and when an extra sword arm is needed in a group.
I would like to see wizard mercs, or any good DPS-type merc. I'm assuming that Wizards would be SoE's choice of DPS'ers since they're pretty straight-forward : NUKE!