Ignoring whatever the properties of the FritoLay (Sunchips) bag is from another vendor, here is the play from Metabolix (if they get FDA approval):
1) Mirel is not a PBA based plastic. At least you will not get cancer from from PBA's if you go with MBLX's products (again, this assumes there is no unsafe level of solvents found in the FDA tests...but PBA's certainly are out of the question.
2) I compost and so do a lot of people but the play is not for individual backyard composters (outside of possibly grocery bags and containers for products at whole foods and etc.). There however are many municipalities that have green waste collection. The bags will compost fine in that (and actually would not require that level of industrial strength composting...the product has very good qualities for composting in ambient temps). The Heritage plastics application of a composting bag for restaurant/yard/road crew waste could be really big as civic composting capabilities grow.
3) This could be very interesting as well. Just from a trash perspective it is a much better thing. MBLX's product will compost on land, in water and even in salt water. Landfills aren't microbe rich so they don't have a great story there (though they have a tiny story in that the will degrade a bit). They have a very interesting story to tell in how products made with Mirel will not survive at sea long enough to be part of those massive areas of waste floating around. I could see Greenpeace, Costeau Society and others want to put their marks of approval on such products packages in Mirel. That assumes MBLX has capabilities in marketing but still it is something that could happen. Regardless of eco-cobranding, there is a great story to tell with the navy if it wants to have trash bags for things that can be dumped in the sea onboard ships.