My biggest concern with it would be this:
The metadata area for files is notoriously fragile, and shouldn’t be trusted with archival information yet. This would indicate to me that this application could be useful for light keywording here and there. For someone that would want to invest many cumulative days of effort over a long period of time, and rely upon the keywords to store non-replicated data, I wouldn’t recommend it.
How many times have you lost Spotlight comments, and would you trust Spotlight comments for important data? They both use the same storage mechanism.