Last year I made a charity quilt with all rectangles, and got brave and quilted it with circles. I used a thread with a pretty strong contrast at the suggestion of a few friends. The big circles weren't too hard, so I got braver and made some random intersecting smaller circles.
I really loved the result! Unfortunately I was running against the clock, and didn't take time to photograph it. Ah well...
Several others have recommended circles. Since I tried it once and loved it, I agree.