I actually picked up my old Merc a few days ago, and specced him heals. Now, I have a heal Sorc, as well as a heal Op, and I will say that I find the Merc the most fun.
I find my AoE heal is always on CD, as I use it so much. It's very usefull, and the added HoT if you stay in the AoE's kolto residue is a great bonus. Your "pew pew" basic attack heals can be used on the move, just like your AoE.
Now, people are saying that heal Ops are the best class, and that you can keep your guy up for ages with Surgical Probe. I will go on record and say, in the hands of a some-what skilled player, heal Mercs can keep themselves up for just as long. You're not as squishy as an op is. You have your shield for defensive. You have a stun, a knockback and quite a few AoEs to keep enemies on the move.
Running away works. You can throw your AoE down in front of you every...What, 10 seconds? And that's not including your quick heal, and healing scan.
So, all in all, I would say Merc healing really isn't bad at all, but is just sorely underrated.