I really want a 3D printer to make free Lego pieces.
Time for someone to stand up for CaptainHurt. He's basically correct. You are not going to get replacement Lego pieces out of a fused filament printer, at least not ones that work with other Lego pieces. Fact is, the plastic parts you'll get out of a fused filament printer are mere representations of what you are seeking. The technology is crude and there are many,many ways that a print can go wrong. Start with the basic process, which is totally different from the injection molding or blow-molding processes by which most familiar plastic parts are manufactured. Fused filament is incapable of reproducing the uniformity and precision of injection molding. It's distinct advantage is that you can make a one-off item. The costs of producing a mold and the runner required to produce an injected molded part makes it an impractical technology for short production runs.
Next, consider the materials. How many variations of plastics are there? Thousands. How many are available as filament suitable for entry-level 3D printers? A mere handful. Will that situation improve? Sure, but the range of materials will still be a tiny subset for the foreseeable future.
CaptainHurt is correct. If you are falling for the hype in thinking that you can buy a 3D printer and start pumping out replacement parts for all the plastic things that break in everyday life, you are going to need to be a patient, tolerant person with low standards of performance. However, if you are looking for proof of concept, prototyping, or working on design projects with students, or you need a special or custom part that does not require high strength, precision or aesthetics, then cheap 3D printers can be fantastic. For art and craft that isn't going to be looked at very closely, they are also great.
I cringe when I see news reports on 3D printing, as they are rarely accurate reflections of the current state of the art, at least that which is available to the masses. I read an article recently in which an "expert" proposed that soon auto parts stores would stop stocking and ordering parts and would instead simply 3D print them on demand. That person was definitely not an auto mechanic, an engineer, or even a technician. Will it happen in the future? Perhaps, but one might as well be writing SciFi.