Ive got one... have access to a makerbot cupcake, own a modified eventorbot, and a printrbot; probably building a rostock derivative soon ish
most arent "there yet" - still largely toys and for "fun" - but they are slowly refining into useable machines. Yes you can print something useful like a cellphone case, no gun smithing isnt there yet (unless your shelling out 20k plus on a statadyne).
If your thinking about the makerbot replicator - do your research. I'll leave it at that.
At the moment, my cnc couter is still a better tool for making things, but at some point - i do expect 3d printers will have their place in the shop...