At first, they sounded just like rounds to me, but as they broke in, they got warmer sounding without losing any detail - just as advertised. The sound is a little scooped, with strong bass and detail and not a big midrange push. It's a little early to tell still, but I'm pretty happy with them so far.
The feel is fairly but not overly smooth - a little more like rounds than flats to me - and the tension is a little more than I'd like. I'll probably try one gauge down next time.
All in all, they seem like pretty nice strings, especially if you're looking for a mellower sound than rounds without going all-out for flats.
The compressors also come with the added feature that they are a bit kinder to the frets of your bass, which might be of big importance if you play vintage basses.