I have a Blackhawk Serpa thigh system. I only occasionally use it for the range and hiking. Primary purpose for it for me is hiking on National Forest land when you have to keep it openly carried, and if it's cool out, a light jacket could cause some partial coverage problems with a belt holster.
Nice for hiking if you're carrying a slinged rifle as well; helps keep it from banging into the pistol or getting caught on the butt of the pistol. That annoys the hell out of me.
I don't get the 'not fast' thing. Puts the pistol at a more natural reach of the arm, providing an easier draw. Perhaps there are exceptions when you're squatted down and scrunched up against something.
And again with hiking, if you're carrying a backpack, vest or something with a waistline belt system, it's also handy because again, it puts the pistol lower and away from your belt for less interference and more comfort.
Some see it as a tacticool range gear piece, but I think it has legitimate uses for some. For those with skinnier legs, some may find it uncomfortable due to the wide paddle-like design of the holster's base.