If you write test functions, you need this. It's like the "Test::More" for "Test::More". However, it currently does not work out of the box with subtests (submitted as wishlist to RT).
PS: Thanks to Toby Inkster for pointing this module out.
Test::Tester is your best choice if you're writing a testing module. Its interface is far superior to that of Test::Builder::Tester. Great work, Fergal.