Unknown, but my experience is that the rate is about 90%, and like I-601's, the rate is dramatically higher if one seeks legal representation.
The above is intended only as general information, and does not constitute legal advice. You must speak with an attorney to discuss your individual case.
I agree with my colleague, but would add that each case is taken on a case by case basis. So the circumstance of one case make it more likely to be approved than another.