Siren is great! I got in pretty early (friend is friends with the people who founded it) and it's been nice to use. Answering questions of the day usually works for matching with people, and the community also puts on some excellent events. One of them had an architect present on the history of erotic spaces, and another presentation on the influence of Leigh Bowery[0] on costume design (seriously, you start to see stuff inspired by him everywhere). It was a step outside the techie bubble which I haven't seen often.

In general I think the sex-positive nature of Siren will help it in the dating apps scene. Recognition of the asymmetry of experience between women and men on straight dating apps is implemented well, including a recommendation system between women. Fills a void between OkCupid and Tinder.

[0]http://en.wikipedia.org/wiki/Leigh_Bowery