I've had the original DP1 since it came out. Got a used DP2 Merrill I've had for 10 days and am still deciding whether to keep it or return to KEH.
I do like the controls better than the old DP1 but I was initially a bit disappointed with the RAW files, in that every daylight image seems to have a slight brownish color cast that needs to be adjusted in photoshop. At this point at least, I don't see the files being any better than the old DP1, unless you're looking to print larger. I'm thinking about trading back for a DP2s which has a slightly wider focal length, would save some money, and is a bit more compact.
I also have a Leica M240 and can say that while the Sigma files have a unique look to them, overall they are NOT superior to Leica's full frame. The Leica DNG color files are usually dead on, with some adjustment optional for personal taste. High ISO performance is good for night shots up to 3200. Not thrilled with b&w conversions for the most part though with the Leica. I'm still a film guy when it comes to b&w.