Mine printed just like yours out of the box. I shimmed everything and tried tightening belts and all the above methods, none made a significant difference.
My problem was that the X axis and the Y axis were not square to one another. While the method mentioned above of moving the carriage to the extremes and eyeballing the gaps between the carriage and the gantries did provide a small amount of improvement, I found a simple construction square to work the best.
All I did was loosen the spring loaded bearing holders so that I could move the bar and loosen the belts. I used a triangle square from Home Depot. I held it tight against the X axis bar in the back [the one you can't adjust] and brought it up against the Y axis bars, one at a time. Gently, but firmly, you can rotate the X-axis carriage rods by pulling on one side or the other and you will hear it "pop" as the belt jumps teeth. That's what you want. It only took 3 minutes or so to adjust, but once it was adjusted, I was printing perfect consistent circles.
worked for me, but your milage may vary.