My initial experience with Gradle is very positive. I've been quickly able to convert over my Maven POMs to Gradle build files, which are wonderfully concise. You can see the last of the results in this commit (work backwards to the parent commits to see some of the Gradle build scripts).
So far, Gradle is living up to its motto: A better way to build.
Next up is dealing with Maven artifact deployments, and the generation of project reports.
I will absolutely be using Gradle for all upcoming projects, and will hopefully revise Tapestry itself to use Gradle at some point soon.