GPL'd code so not usable by most commercial projects
The bigger limiter, mentioned by other comments, is that it downloads every page in the rss feed to do the search. So it's workable for a smallish niche type site with a low page count, but not workable beyond that really.
That said, there's no way (yet) to DRM or otherwise prevent the user from obtaining the entire front end source, so it might be moot.
Side question: what am I allowed and not allowed to do with webpage sources under US copyright law? If I cache the site and open a local copy, have I illegally reproduced a copyrighted work? What if I burned it to a CD? What if I e-mail a copy to a friend? What if I host my own version?
> cached locally
That falls under US fair use laws I believe
> burned a CD
Again, fair use I believe
> emailed a copy / hosted own version
Likely is infringing, but not definitively. Distribution is generally not fair use, but there are some exceptions.
You can't say anything definitively falls under fair use. Fair use is a case by case determination by a court limited to each specific instance (so if you do the same exact thing a second time it could be found to no longer be fair use).