One of the problems with 3D printing is the build process- the laying down of material on top of previous layers a bit at a time. A bit like a pottery beginner making a pot with a clay snake, except that the plastic is liquid when it comes out of the nozzle. Feed it too fast, and it squidges. Too slow, and it laminates because it was too cool to stick to the previous layer. And you can see the lines where the layers were made. The cheaper the machine, the more pronounced the problems. Here's a link to someone who has used 3D printing for model trains: 3D printing discussion

Some of the results look excellent, others rather crude.

Their comment:

One final thought, 3D printing while becoming more affordable is still expensive and most (at least all the affordable ones) processes still leave a visible stepping which, IMHO, would be a dissuading influence for most potential customers. Post cleanup/corrective action these can be use very successfully for resin casting though. One exciting thought is that some companies are now experimenting with 3D printed injection moulding tools. The quality is questionable and the process is too expensive for the likes of myself but it is showing potential...