I've been using the http://www.ruby-toolbox.com popularity score as a similar type of proxy to usefulness, reliability and desirability for ruby gems.

It attempts to sum up how widely used and active a particular gem is. Plus the site categorizes gems, so when looking for a solution to a new problem you don't have particular expertise in, you can see at a glance which gems may not only help you out, but also those you can trust will be maintained for awhile.