Sold the s100 for the rx100, the rx is significantly better at lower iso's, at the highest isos the advantage didnt seem as great, although I cant recall if this was due to jpg in the rx vs raw from the s100(due to lightroom not supporting the raws for the sony yet).
performance is way way faster too, its almost instant autofocus compared to the s100, I think street shooters would love it.
I think its real downside is sony interface, Ive always felt very at home with canon's ui. I had a nex before both cameras as I thought like you it would be a nice complement to my dslr, and also ditched it due to switching lenses/still too clunky. The nex really was awful to use, the rx is better but it takes some getting used to. Also the rx100 feels solidly built, but its just not ergonomic compared to the s100. Its like they took all the specs of the s100 chassis but then got rid of all best bits, the nice grip and the tactile buttons, and put in smaller fiddlier controls, it definitely doesnt feel as good to handle as an s100.
I think its worth it if you really care about the image quality, to me it fits quality wise between 7d the s100. If you are happy with the s100 produces, it may not be worth it.
The tipping point for me was that the video it takes is leaps and bounds better than the s100(with af during filming).