They're great! No problems so far reusing the same ones for the past year.
I'm keen - no chance of bottle bombs or any of the odd things that can happen to glass, lightweight so easy to store/lug around. Cheap too.
Not as nice as drinking from glass, but I guess most people are pouring into glasses to avoid rousing the sediment in the bottles, so not an issue really.
I also find the nobbly bits at the bottom of the plastic ones are good for catching yeast â sediment seems to stay packed into them more, giving me a slightly clearer beer than I get with glass.