In the CPAN modules I maintain, three use both ExtUtils::MakeMaker and Module::Build and the most recent only uses Module::Build. I find Module::Build easier to work with and it has advantages that I use. I don't like having to maintain and update both of them, so my most recent module only has Module::Build.
To improve my testing I use several optional modules, and use the build process to probe for and only run tests that can be run - rather than getting the tests to skip.
People doing smoke tests on my modules report false positives because they try to run tests that can't be run. I'll put steps in place to prevent that from happening, but in the mean time I wonder do people think that both build processes will hang around in parallel for ever? will one replace the other? which one will become "standard"?