We've since rewritten the system and, while we still support an optional effects based style in our DSL, it's not being used very heavily. In practice, our user found the ergonomics of the effects system quite challenging, and it introduced some type inference challenges. The biggest problem was that our use-cases ended up with functions that would have hundreds of effects, and it was fairly unwieldy and the type errors were difficult to deal with. Since we've introduced the updated version, most users prefer to user our newer features that allow them to write more traditional code even though it means they don't get composable effects.
On the other side of the experience, I've come to believe that effects systems are a good idea, but when adding them to an existing language it's probably best to make them an opt-in feature that can be used to constrain specific small parts of a program, rather than something that should be applied globally. I also think we need a bit more research into the ergonomics before they are going to appeal to a lot of users. That said, the guarantees and optimization opportunities ours gave us were really nice, and were quite difficult to achieve without building on top of the effects system (our new system is about an order of magnitude more code, for example).