I find myself spending a lot of time testing hacks before I can even consider implementing them in our production Trac. Often there are a number of similar hacks to accomplish a particular task, and I end up testing more than one.
The number of open tickets is one way to gauge the quality of a plugin. Other useful ways to do this would be to have a poll in the NewHack template, such as the PerforcePlugin#Feedback has.
If download statistics were available on TH.org, that would be another interesting way to determine the popularity of a hack, which sometimes implies quality.