I bought some last fall, and I actually wore mine today!
For what it's worth, I don't think I've ever had any muscle soreness from wearing them. However, I didn't wear them all the time last fall. I WILL say that they're very comfortable on the feet, and when I wore them today I could feel my hamstrings when I walked (they were sore from running two days ago), when I normally feel my quads--so there might be something to the claim that they work your posterior chain.
At best, they're a workout for your legs. At worst, they're a pair of comfortable flip-flops that take a bit of getting used to