I've developed a serious interest in the Foveon sensor and the DSLRs that Sigma has put it in.
Here is my interpretation/summary based on what I have read:
1. Under the right circumstances, the Foveon can produce images that can knock your socks off - definitely a "3D effect" of sorts to be felt on some of the best shots. I don't feel I have seen this in any 35mm format - when I look at some promo shots from medium-format cameras I see something similar.
2. The operative words are "under the right circumstances". It doesn't do so well in low light and handles noise poorly.
3. The Sigma bodies and processors using the FOVEON are a little behind the times in terms of speed. The processor isn't too snappy either.
4. Overall I think it takes some getting used to before you hit that sweet-spot in terms of subjects and conditions, before you really create those "wow" images.
5. I think it's a necessity to shoot in RAW to harness the dynamic range that the sensor can manage.
..but this is purely a very personal and subjective interpretation. I have never shot with a FOVEON sensor.
Nevertheless - purely based on the images I have seen it produce - I may very well jump ship and get the upcoming SD-15 if the reviews show that some of the sluggishness and the poor ISO handling has been improved.