Agree, the definately most powerfull gallery out there and it is GPL.
Trought it is smarty and very modular inside. If you like a powerfull something you would configure buy turnng on, off and ajusting the options it is nice. But any other change like adding own options, changing the gome page into something else, adding a contact form or even deleting its native ones becomes a tricky task as there are hell of files and deppendancies under the hood.

It mihght be a challange to teach theclient how to manage it (well... even for you it sometimes might be a challange to remember "where was that contorlled by")