The PET are a great short term storage bottle. LOng term, there are issues about oxygen permeability of the plastic.
I have picked up old unopened PET soft drink bottles and they are flat, so this could happen to your beer. CO2 out, oxygen in.
Short term, they are great for posting to comps, or taking in the esky. I have a dozen or so.
Long term, I use glass.
If you reuse clear PET bottles, make sure you protect the beer from light to stop skunking.