I echo what Peter said. I enjoyed intercooler but found Unpoly fit my development style better.
It's really hard to imagine how v2 could improve what was already so good, but I've shipped one of my own apps with the v2 RC and the upgrade from v1 to v2 was smooth with no changes to the markup. I had minimal changes to remove the upgrade shim, and then even added new functionality using the stackable layers. It's working fantastic so far. This is my second smallest app with Unpoly, so it's not the best example, but I'm confident my other apps will be updated in due time without issue.
As far as the stability of the release candidate goes, it's my understanding that its API is finalized and the only remaining work is documentation. Looking through Henning's notes and some of the source code, that seems accurate.
Adam