They are not actually a new product!
3D printers have been around sice the 80s, and the 500,000 number does include printers sold from those early days, but the industry still expect to see the millionth unit to be sold sometime in 2017, and they expect to see more than 1 million units sold per year starting 2019.
Not everything is going well
According to data from CONTEXT, an IT market research company, the first half of this year saw a very mild decrese in 3D printer sales (1%) after a 29% growth in the sector between 2013 and 2014.
Industry heavyweights Stratasys and 3D Systems are also struggling with revenues declining 18% and 9% respectively this year. Their situation may actually get more dificult in the short term as big name brands with deep pockets like Canon, Ricoh, and HP are entering the market.