I built a RepRap Prusa Mendel last year, and boy was I ever in over my head. I was totally green to everything, and it was quite a challenge. The initial kit cost about $800 with shipping, and 16 months later I am approaching the $2000 dollar mark in replacement parts, electronics, and tools. Don't get me wrong, the thing has been a great long term hobby and I have learned an enormous amount about electronics, arduino, and particularly how tedious it is to keep a precision machine like this in tune. But that is what the printer was to me - a project in robotics. It has not improved my day to day experience in any meaningful way outside of that, for a very particular reason.
I found myself in a curious place about 12 months in. I had taught myself basic electronics, taken courses at a local hackerspace, and finally got everything working like a charm. I printed non-stop for a few weeks, and then found that my hobby had dried up. The printer was done, and I lacked the very most important skill for operating one: the ability to make my own accurate and printable 3d models.
I have been learning, but I wish I had gone about this in the opposite order. Hackerspaces, Makerbot stores, online services like Shapeways, and even some libraries now have properly configured 3D printers available. If you are thinking about getting/making one, I strongly recommend learning the modeling first and testing your prints with these services, and if you get good (and fast) enough that this becomes a hassle, then you will find your printer truly satisfying.