How does Ricoh GR compare with Sigma DP1 Merrill in image quality, autofocus, lowlight performance and ease of use?
There is literature online regrading this debate. You can simply google, "Ricoh GR vs Sigma DP1".
I do not own or have used a Sigma to make a direct comparison. I can only speak for the Ricoh, and it 's one of my favorite cameras of all times. The Ricoh takes fantastic quality images, but best the part of the camera is the way it handles. Ease of use and menu layout rivals a Leica. AF in low light is good when the AF assist light is on. Images are quite good up to ISO 6400, and degrades after that.
Some of the best pictures I have taken have been with this camera. I prefer this camera over my Canon 5D Mark 3. I would buy this camera many times over.