Could you please elaborate on the current state of the project? Can it be considered stable? Where should I look for the uses of extensible effects?
Ben Foppa has already answered most of these questions. I should add that although there are some improvements to be made (I'm thinking of a couple), they are either definitely or highly likely will preserve the existing interface. So, your code will not have to change. For example, closed type families in the upcoming GHC 7.8 will let us implement OpenUnions better. The interface shall be preserved, only the implementation will change. Most of the new development is writing more examples and more and better explanations. Recently, for example, I added committed choice (in Prolog terms, soft-cut) to the Choice effect. I was surprised how easy it was and how nothing had to be changed. I merely added a new handler. So, we get another implementation of LogicT, this time in terms of extensible effects.