It depends on your music library, but bliss will typically match in the low 80%s range for a 'normal' music library consisting of mostly mainstream releases and some more alternative recordings. I aim for accuracy of at least 95%. As bliss is automated I tend to concentrate on accuracy first.
Furthermore, each release of bliss is "stress tested" using a large, diverse music library. By doing this for each release I can keep track of how successful bliss is at finding artwork and its accuracy over time. In addition, a large scale bliss cover art lookup test is run using album and artist names sourced from the Rock and Roll Hall of Fame.
Any other FAQs I forgot or clarifications required? Post your ideas below!