Supposedly, this unit was built off of the work of the Open Music Labs x0x-Heart module, which is open source. That being said the TD-3 sounds the part of a 303. Of course it's not 100% like the original, but nothing will ever sound like the original and the originals all sound different anyways.However, for the price, there are zero valid complaints in regards to sound. The unit sounds the part and does it well on a budget.
Unfortunately, the sequencer also fills the part. It's archaic and cryptic just like the original, which is what led to the downfall of the Roland TB 303. Thankfully, Behringer made an easy to use computer sequencer program that is quick to get good results. Just connect the TD-3 to the computer via USB and you're off to the races. I was able to use the program without any instructions in stark contrast to the onboard sequencer of the TD-3.
Everything else like build quality is good. The machine is light, but that doesn't correlate to quality as it's probably just a single circuit board and no on board power supply. My unit did ship with a UK power plug to the US. A slight inconvenience, but I think Behringer and Thomann could eventually offer country specific plugs worldwide. The distortion circuit does not sound great, but it could be used to get extra sounds out of the machine so a welcome addition.
Overall, I would recommend the TD-3 to anyone that wants the 303 sound. For the price, there's nothing else out there that comes close and also is analog. It sounds better than the Roland TB-03 digital reissue (DOH!) for way less money. The alternate sequencer on the Roland TB-03 is better though. My answer is to get both. The TD-3 for the analog fatness and squelching filter, and the TB-03 to fill a different sonic space that digital does well. They would complement each other well.