A recent LiveScience magazine article explained doubts about the environmental credentials of compostable products, using various life cycle studies to make the case.
In simple terms, the environmental impact of these products depends on whether they end up in a landfill or the compost heap.
In a landfill, conditions are generally anaerobic [i.e., without oxygen], so compostable materials do not 'compost'; instead they either persist or break down anaerobically releasing methane, a greenhouse gas that is about 30 times more potent than carbon dioxide. If these materials land in a compose heap, they do fare better but it depends on the composting setup of the facility. Another study found that compostable dishware items that were composted tended to have higher environmental impacts across their life cycles than non-compostable equivalents.