yeah i went last saturday (not last night, the one a week ago) and also on halloween. it was nicely busy on both occasions and a very friendly vibe. i personally prefer the basement much more like this than in summer, when it's rammed.
hard to guess a number really, but it definitely has a pull at the moment and it's just something new for the ibiza winter clubbing circuit, which is great. there were enough people there on both occasions to create a really good vibe - and that's all you need.