Tests with the old 4.7Mpx Foveon showed it to resolve at roughly twice its dot count compared to a conventional sensor with a Bayer filter, for example:
Sigma SD14 Resolution: Can it Hang with the Big Dogs?
Not quite the 3x that Sigma would have you believe.
Since there's only one company manufacturing the things, there isn't the rapid pace of advancement that you see with CMOS sensors. The current DP compacts are using technology that's close to five years old.
The Merrill 15.4Mpx sensor is quite a bit newer, of course. However, it lags behind current CMOS sensors in a number of ways. Reviews of the SD-1 show poor high ISO performance, for example:
Sigma SD1 / SD1 Merrill In-depth Review: Digital Photography Review
I don't think I'd consider a Sigma DSLR even at a much lower price than they sell for - you'd have to really, really like Sigma lenses. The forthcoming DP Merrill compacts are rather more tempting. Assuming the sensors still punch at twice their weight, that would be a 30Mpx equivalent, large sensor compact that fits in your pocket. The 30mm would make a pretty decent carry-round camera. Were the fixed lenses F2 rather than F2.8, they'd be almost compelling.
I don't know what the prices will be like either. The old DP models were expensive for a fixed lens cameras. I can't see the new ones coming in below a Fuji X100, for example.