We use some industrial directories such as ThomasNet, MacRAE's and GlobalSpec. These directories do provide some, but decreasing value over the years. Google (according to Matt Cutts) looks at a few items when assessing these directories:
- Editorial discretion (who can and can't get in)
- Value (original and informative content)
- How much work they do on the site
- Usability
A good rule of thumb-if the directory looks outdated, lacks original content, has broad content (not in all cases but niche is better), looks spammy, has poor usability and anyone can get in don't use.