Here's a story. I always wanted to make a CNC machine. I got a bunch of parts off Craigslist from a guy, got some stuff off eBay, I was in college at the time so I never really had time to do anything with it (and money was tight), so I couldn't get it finished. But in college I saw some 3D printers, decided I wanted one.
Moved back home with my parents, christmas / birthday was coming up. I was looking into some printers (I was going to get the Solidoodle 2) to get. I was talking to my friend about it one night, said to myself "Whatever, I'll just make it!" (not sure why that thought popped in my head). 6 weeks after that, I have this thing: http://imgur.com/a/FZfp9 in my room. Wish I would have documented the build process better.
I wanted one because I'm making a cool product and needed to make some cases for it.
I use LinuxCNC to run it with a HobbyCNC stepper driver board, and it actually works pretty well! If I could make one 'enhancement' for it, I would make native gcode arcs work in slic3r, as I can hit 60-80 mm/s on a straight away, but around corners it gets to ~25 mm/s.
I've found two 'killer' uses. My nieces and nephew come over, they want to make something (my nephew asked for keys one day). We pulled up thingiverse, he picked one out that he liked, 45 minutes later he has keys.
The second one is being able to design something and see it almost instantaneously. Is this going to fit? How does this look? Would it work better this way? You don't need to sit and twiddle your thumbs for so long, as you can test out a ton of ideas.
One of the pains is the calibration for this thing - I made this script (which definitely needs some work) https://github.com/MercuryRising/dic3r to aid in calibration. You can calibrate your build plate (it makes a skirt of your platform size, so you can even it out while it's running) and I think it does extrusion multiplier calibration. The one thing that I think is really cool for it is variable density (specify a Z height to merge two different slices of stl). You can make stuff like this: http://imgur.com/a/B26e5 with different infill patterns.