I feel that the s2 is somewhat underrated. Or maybe underappreciated is a better term. I think that many of the reviewers gave the s2 a mediocre rating without real justification. And I think that in many cases it comes down to user bias. Which would match my own experience.
I owned the 650 for 10+ years. It was my "normal" and the headphone I used as a baseline when testing new headphones. I put it on a pedestal, having put thousands of hours on it. When I picked up the s2, I liked it, but didn't rate it as a successor to the 650. As time went by I suppose my bias lost out to the actual performance and enjoyment of the s2.
Now I make no mistake about it. The s2 is a better sounding and performing headphone than the 650 IMHO. I suspect that some of the reviewers who gave the s2 an average review, had a similar experience that I had...maybe.
Regardless, the s2 is now a daily driver for me and my favorite of the Sennheiser 6x0 lineup.