First, it all depends on what type of plastic it is made out of. Some plastics leach oxygen making bad beer. The plastic that fermenting buckets are made of food grade HDPE IIRC and leaches oxygen but very slowly which is ok for a week but not ok for long conditioning or lagering.
Second, Better Bottles uses a special PET in their carboys that they leaches no oxygen and resists adsorption. They say it is taste and odor free. I have not seen anyone report of any plastic taste or problems with these carboys.
Third, beer is sold in plastic primarily so people can take beer where glass is not allowed, I think this plastic is food grade HDPE as well. Could you imagine rowdy fans and glass bottles? Bad news.