I recently read @apemberton article comparing trends for Maven vs Ivy. I wasn't surprised to see that Maven was a clear winner and even less surprised to see that the Google trend line was still in a positive direction.
However, I decided to add in a couple of other Java build tools as well - in particular Buildr and Gradle. This is where I was surprised - Gradle surpassed Maven in terms of interest in the across the intertubes.
Now this doesn't indicate adoption or maturity, but clearly over the last 2 years, Gradle has been on a steep upward trajectory. This will surely translate into more adoption for enterprise projects and is definitely a tool that I need to spend more time learning about it.