A 3D printer is great for people who know CAD. I love my little Afinia. Sure, I have printed out my fair share of toys and models. But where it comes in handy is when I need something very specific to solve a problem both at home and work.
For instance, I bought my grandma a weather station for her birthday. However, there was no place to mount some of the sensors. After about a half-hour of design work and about 45 minutes printing time, I had some quick plates that I could glue to the shed and screw to the fence posts to mount the temperature and wind sensors. Sure, I might have went to the hardware store and looked for a solution, but that would have taken much longer than the hour and 15 minutes it took me to design and print something.
Simply put, if you know CAD or are willing to learn, 3D printers are great. If you are only going to print what other people have made, don't bother. After the toys and other tat, you will lose interest and probably feel like you wasted your money.