I discovered the world of 3D printers when I was looking for interesting projects to build with Arduino, as I'm starting to learn electronics to augment my coding skills. I could have started with a programmable LED cube, but when I discovered the RepRap project I was immediately hooked. Not only could I learn a lot of useful skills and get soldering practice, but at completion I'd have a machine I could use to build custom cases, buttons, and any other custom parts I might need for future projects!
I did a lot of up-front research comparing the commercial offerings to a growing plethora of open source designs. It seemed clear that I would probably save money and gain more valuable experience by self-sourcing parts and building a RepRap myself, so it was really just a matter of choosing a design. I finally settled on the Prusa i3 Mendel for several reasons, foremost being the large (20cm^3) build area, low cost, and elegant design.
The sourcing and acquisition of parts took me about a month, and I made some noob mistakes, such as buying unsuitable stepper motors and the wrong RP parts for my chosen i3 variant (single plate). I also needed to buy tools and supplies, such as a soldering kit, a grinder to cut metal rods, a glass cutter, nuts and bolts, Kapton tape, blue painter's tape, acetone, etc. My main sources were SeeMeCNC, eBay, and McMaster-Carr. I overspent a bit up front, but I was able to recoup most of that overage in reselling my surplus. Today I have much more savvy (and now I can print my own parts) so I could easily build a sister to this printer for under $500.
I did learn a lot in the process of building this machine, and I've learned a lot in the process of enhancing and upgrading it since. The printer certainly hasn't "paid for itself" even a year later, but that doesn't matter to me. I did this project to educate myself and get hands-on experience, and compared to the cost of a college semester it's been a total bargain. Not only am I now familiar with Arduino programming (and have contributed code to Marlin firmware - you're welcome), but I've gotten pretty good making things in OpenSCAD, gotten to know a great group of geeks at the Seattle Metrix:Create Space, delved into Blender 3D, and gotten to know electrical current and the smell of burning components... None of which I would have gained just buying an off-the-shelf Cube3D.
The progress of low-cost 3D printing has really been accelerating lately. Some of the most vexing problems (such as bed leveling) are being solved, better extruders are being made, the slicing software is smarter and faster, and the quality of parts designs is constantly improving. I've got a half a grocery bag filled with failed prints and imperfect prototypes after a year of messing around with this machine, but I've gotten really good at calibration at this point, so very few prints fail now. You do still need to watch prints carefully, and that goes for the commercial machines as well, but generally speaking the reliability of newer machines is much better than their predecessors.
As for how useful a 3D printer is to any individual, that will depend on the intensity of their interest. I took the time to learn OpenSCAD, but not everyone will feel inclined to do so. I've made some useful items, such as the "hanger" part to repair some Sony headphones, a light cover, a slick sign for my workshop door, some iPad sound deflectors, cases and covers for various things, and of course upgrades for the RepRap itself. I've made several sets of printer parts and sold them on eBay, so the printer is slowly paying for itself. I help others with their 3D printer builds, sharing the experience I've gained in my first year, and that's a lot of fun. I think it's a great tool for hobbyists and professionals alike, especially those with engineering skills, and I can anticipate a time, not too far in the future, where 3D printers will be as ubiquitous as home computers.