PR of the directory is just one of the many elements that make a good directory. See if -
1. Directory and all its pages are indexed in Google, without the Supplemental Results tags.
2. The directory is linked from few high PR pages or many average but relevant sites. Latter is better.
3. See if the directory ranks well for any of the phrases which it apparently promotes. Few directories that had high PR weren't even ranking for their own names :)
4. Quality of editing. Are the sites in neighbourhood are related to each other? Do editors allow you to use just keyphrases as Title? and many other signals of quality.
5. Age. An older one that has weathered the tests is perhaps a reliable one.
6. The directory apparently uses some readymade directory script? Quite common when you are unable to distinguish between directories that use similar script.
7. A "Similar Pages" search in Google shows other directories. Lets you know if the directory is looked at by Google as a directory at all or not and to some extent lets you know if the directory has a network of other directories which may cause duplicate content, interlinking problems.
8. Is the directory just a collection of links or does it have some unique content also?
9. Is the directory of the type that believes in having all the categories up, leaving most of them empty?
The Contractor has given some valuable tips here in [webmasterworld.com...] msg #:102.