I use this for a majority of my sidechain compression, it's very nice and can sound both vintage or modern depending on how it's configured. Only downside is that the release sometimes (though rarely for me) can't go low enough.
I bought the Waves API bundle during a Waves promo after reading the review of SOS (without even properly demoing it). When I put it on the drumbus of the track I was working on, it instantly put a big smile on my face :-D
I still have it on (almost any) drumbus and after all these months it still puts a big smile on my face.