The only sockets I've had fail, are ones that are in the "occasionally used for high power" category, and it's not an unusual scenario for failure. They all seem to be a cheap brand, installed by the previous owners, who I'm fairly sure got one of their fathers to rewire the entire house (he appears to have been an electrician, so nothing specifically dodgy per se). If they did the rewire with the expectation that they'd sell, then I'd expect them to have used relatively cheap parts, and my experience has borne that out.
I've ordered a small pack of Crabtree ones, to see what they're like.
I'm still unsure about the USB sockets. Most of the points raised, are what worry me, mostly having the device, in effect, permanently on, and the associated risk of fire, especially smouldering away unnoticed for a while. Power use, as others have said, is likely to be small, and probably dwarfed by things like the fridge, freezer, NAS etc. As it is, I already have USB power supplies, almost permanently plugged in, so there would likely not be a dramatic change.
Given the price, I'm not going to rewire every socket with a USB version, but likely a couple by way of experimentation. In an optimal world, I'd build my own, with plenty of redundancy and tolerance, but that would (i) be quite expensive, and (ii) unlikely to fit a standard patress. Part of the reason for using them, is to minimise significant adjustments to the house wiring.