They're fine.
I bought 24 Youngs I litre PET bottles for about £12 recently for 'going away on holiday' beer, and they hold pressure very well during conditioning. They are pressure-rated way beyond that of glass beer or even champagne bottles, and of course you can get an idea of how conditioning is progressing by the 'give' when you squeeze them and even crack the tops to vent them if you think the pressure is getting excessive. They are designed specifically to be used over and over again.
I am also in the process of building up a stock of 5 litre water containers. These make brilliant demijohns if you drill through the caps to accommodate an airlock and are of the ideal size for lagering if you don't (I can only get 1 23l FV in my fridge but I will be able to get 8 or 10 water containers in). The 2-litre pop bottles look very attractive too - especially when 4 bottles of 'poor people's lemonade' can cost less than a quid. Down the sink with the rubbish lemonade, keep the bottles. Job done.
The plastic bottles for pop, water etc are made of precisely the same material as the Young's plastic beer bottles - Polyethylene Terephthalate.
Ignore storiess about chemical extractives nowadays - if they had their origin in truth (which I have no doubt they did), it was many, many years ago. If there was ANY chance of noxious substances contaminating your brews from food-grade containers in 2011 , our health-and-safety-conscious bureaucracy would have them banned before you could say 'paranoia'.