This is great from an environmental point of view. Glass is heavy, leading to increased emissions during transportation. While it is recyclable, here in the UK we import huge quantities of green glass in the form of wine bottles, but we don't have the need for it in our own packaging industries leading to a geographical imbalance. The plastic bottle is made of 25% recycled content and could either be recycled locally into a different form or exported much more efficiently.
So what's the hitch? Well, frankly, it feels a bit odd pouring decent wine from a plastic bottle. It squeezes in your hand and you don't get the same impression of quality that the solid heft of a glass bottle gives. So it may take some time to get used to it.
Wisely Marks & Spencer have chosen to market the bottle as "shatter-proof" and particularly suitable for outdoor dining - turning the potential negative into a positive. This gives them a chance to get the product out into the market and get people used to the new way of doing things. A sensible tactic worth considering for any green product.
BTW: Marks & Spencer and their Plan A sustainability programme feature in my latest book, The Green Executive.