I built a RepRap Prusa i3 a few months ago from a kit with my son. I shopped around and found a good source. The kit had an aluminum frame (there are also wood frames which are a little less expensive) and was sourced from a person who runs workshops, so I had confidence that it would have most of the parts I would need. I bought it on eBay for $680, though you can find them cheaper.

There are probably better printers out there. I've heard good things about the Printrbot. A friend has one of the earlier wood framed ones. He likes it. We saw the new metal ones at MS Build a couple of weeks ago and the prints were excellent. The reason I chose the RepRap is that it's open source end-to-end: hardware, electronics (Arduino Mega), firmware, and software. We've had good results with Repetier Host and Slicer. I like the ethos of the community and the idea that it is (at least somewhat) a self-replicating machine.

The main thing about 3D printing is that it's a constant effort to fiddle and tune. It took us a couple of weeks to get it printing well. We printed some replacement parts and printed enhanced parts--for example, a pulley tensioner.

One of our early challenges was getting the right surface for the initial layer. The printing bed is glass, with a heating element underneath. The heating is optional and is mainly used with ABS plastic (petroleum based). We use PLA (vegetable based), which doesn't release the noxious fumes. It's probably the most common plastic used in consumer 3D printing. Many people use blue painters tape on the glass. The plastic tends to adhere pretty well. We tried that and moved to PET tape. There are lots of resources for tweaking your recipe here.

For filament, we use 3mm diameter, because that's the type of hot end that came with our kit. Most printers are now using 1.2mm, which is thus more readily available. The MakerBots use that, so you can even find it in some retail stores. We might print out a new extruder and get a 1.2mm head, but our extruder is working well, we have a good source for consistent spools (MatterHackers--great service from them) and so I don't have a good reason to change right now.

So it's a constant effort to calibrate, tweak, and tune, but it's been worth it. My son has it running all the time. We've gone through several pounds of plastic. We download most of our stuff from Thingiverse, but have also designed and printed our own custom items. I'm very busy, but it's a very manageable hobby. In our case, the big time investment was two long weekend days to assemble it. I can highly recommend it if you have some technical aptitude, patience, and a little extra time. It really does feel like we're seeing the future. Being part of it is pretty fun.