I have never gotten into the mech debate before, but here goes:
I think the debate boils down to preference. I ditto what PABowhunter said about you still have to have a tuned bow and at least 55 lbs of KE. But there are some not so good designs of expandables. The newer designs have improved dramatically over the past couple of years. The ones with the larger cutting diameters require more KE so I would stay away from them unless you have over 65 LBS of KE.
If you have a tuned bow, have( I prefer at least @60 lbs of KE or more), and a cutting diameter of less than 1.5 inches, I don't see any problems. I use the Shockwave because the o-ring is behind the blades (less energy to open). It also has to muzzy type head to bust bone. I shoot about 63 lbs of KE and I have killed deer in AL, KY, GA, IL, and MN with mine. Still have yet to have any problem and passthroughson all exept one, it drove into theground before it could pass through.
If you use mechs, stay away from the big cuttingdiameters and make sure you have enough KE. I dont buy that the newer mechs dont work all of the time.The way most of them are designed, they have to work if you only get 2 inches of penetration because the blades have to open upon resistance, no way around it. A bad shot is a bad shot!, no matter what you shoot.

My advise: Shoot what you are comfortable with, If it is mechs, have the rightsetup and pick your spot. Maybe if we all concentrated more on shot placement, the debate over fixed vs. Mechs wouldn't be that big of a topic.