There seems to be some concern about off-gassing and chemical taste in sensitive people. The jury is out on this because there are no studies done to test silicone's reaction with food under heat. FDA says silicone bakeware is safe, but they also say MSG and nitrates are safe, and have failed to ban artificial colorings even though they've been banned in other countries. So I don't put a lot of faith in what the FDA deems safe.
There are some warnings that lower quality silicone bakeware contains fillers that could be hazardous. If you’re set on buying silicone cookware, check with the manufacturer regarding this. To test for filler yourself, one suggestion I came across on a few sites was to twist the item and if white shows up on the bend, the piece likely contains filler.
Many silicone baking mats are made of fiberglass covered on both sides with silicone, so unless you want to risk fiberglass in your food, don’t cut on the mats.