To people worried about fragmentation, retaining binary compatibility means it really doesn't matter. Scalaz can be written in the Typelevel fork and you can use it without issue from code compiled with the standard Scala compiler.

Also, from the announcement:

"As a conservative fork, our aim is to be merge compatible from the Typesafe Scala compiler, and a key objective is to feed pull requests (many of which will resolve long standing Scala bugs or feature requests) back to the Typesafe Scala compiler."

So we aren't looking at radical changes, and hopefully these changes will get into Typesafe's compiler.

So basically it's an opt-in better Scala compiler with no effect on compatibility. What's not to like?