What differences actually show up between the true-color Sigma Foveon X3 sensor and a conventional Bayer matrix RGGB sensor?
This question speaks directly to real-world usage and hence to camera choice for certain purposes. There are other factors too, but seeing the differences can help clarify a choice when making a buying decision.
The Sigma DP1 Merrill and Ricoh GR are well matched in sensor size and resolution and lens field of view and performance, and neither has an optical low pass (anti-aliasing) filter. This scene turned out to be a great showcase for what the differences can be with real-world images.