One large bug: it won't let non-registered users view detail images, even when that choice is set to "yes" in the backend.
No support: Despite numerous requests, my questions on the JoomGallery forum have gone unanswered. And there doesn't seem to be any documentation, at least not in English.
Conclusion: This could be one of the very best Joomla! extensions if the bugs can be ironed out. Until then, just mediocre.