Traditionally to produce your product you had to go via the mass production route. With 3D printing — and digital manufacturing at large — you have a second affordable option available. Of course it depends on the product itself, the components it consists of and the expected volumes which production method makes sense.
3D printing makes sense for one-off and small series production. That is the obvious answer. The more elaborate answer is that it depends on the actual volume of the product and its parts. Redeye (division of Stratasys) made some calculations and put them on the web. They came to series of between 100 and 250 parts. It depends on the material and complexity of the part but in essence when volumes go up mass production methods like injection molding start to make sense. I put in chart to make it more tangible.
The setup costs of mass production methods make it prohibitive for small series production. If you take injection molding the tooling cost start $10.000 and up. The setup costs for 3D printing are virtually zero. That makes the difference.
Other considerations are material, part complexity and source location. 3D printing has a limited set of materials and may or may not suit the requirements for your parts. The complexity of the part is also important. More complex parts require more tooling costs while with 3D printing the cost impact of part complexity is low. 3D printing even allows to make parts which you cannot make using mass production methods. But that is something for another blog post. The last consideration is source location. 3D printing is available around the corner in most Western countries and thus be sources locally effectively saving on shipping costs and lead time. Mass production typically takes place in East Asia because of lower costs.
In small part series 3D printing can be a viable option. I expect prices of 3D printing to go down significantly in the next 5–10 years effectively raising the bar for traditional production methods. Maybe 3D printing will even replace traditional manufacturing methods all together.