Maven's main strength is standardization of the build combined with the dependency management that makes it easy to get a build going instantly without having to explicitly download libraries.

"slow" - can't say I've found that an issue when compared with Ant which I used to use.

"verbose" - in configuration I disagree - the more you put in your pom the less well you're using it. In output true, but I'm not sure why that's intrinsically a problem for you.

"inflexible" - absolutely not true. You can configure a plugin and Maven will go and download it for you - so as a user you can enhance the tool directly from the configuration file (e.g. I typically use a plugin to build database scripts from annotated classes).

"opaque" - yes, but there's method in that madness. It's programming by exception (how is my project different from the default?") as opposed to Ant's (mainly) explicit programming ("How exactly is this build bootstrapped?") Once you know the pro forma you can jump into any Maven project with an immediate understanding of its likely structure - very different from Ant where you basically need to read the build.xml files to know where everything lives.

"thinly documented" - Yep. The documentation is horrible and that's its biggest problem in my opinion.