A double boiler can be a great tool in the kitchen, whether you buy a “real” one or use a stainless steel or glass mixing bowl set on top of a simmering pot of water. I do not recommend using a silicone double boiler, like the one pictured here.
Silicone is a great material for cooking utensils because it is heat resistant and flexible, so a spatula, for example, will work equally well for folding egg whites in a mixing bowl and for stirring a stir-fry. It also makes it a fantastic option for potholders and other heat-handing tools. This heat resistance, however, does not make it a good choice for actual cookware – including double boilers – because cookware counts on heat transfer from the stove-top to your food (this is also true of silicone bakeware). Silicone will transfer the heat, but it will do so much more slowly than other materials. This could be a benefit if you don’t want to stand over your double boiler and stir your tempered chocolate (or other mixture) while it cooks, but the whole process is going to take much longer than it otherwise would.
If you’re going to get a product like this, I would recommend keeping it as a mold for other foods (jello, dome-shaped cakes or summer puddings) and not for use as part of a double boiler. Stick with a traditional material to ensure that you get the best and most efficient results possible.