Yeah, I've seen people that do not like PX200-II seal and fit. But on my head they sit perfectly and isolation is almost as good as with some of my IEMs. Sound leakage is almost negligible, and overall SQ is great. So, you just have to try them on your head before you buy them, I think
As for the worst headphones I ever bought... I think, considering price/quality ratio, that would be Sony MDR-EX310LP, my last IEMs. For the price of ATH-CKS50 (which I had before), EX310 souds absolutely awful. I'd say they have V-shaped sound, but that "V" is stretched vertically a couple times, so you get loud, muddy bass and creaking sibilant higs. And cable is cheap and bad too.