I've been using Gallery2 for quite a while and it is quite powerful - which makes it pretty heavy and can be slowwwww once your database starts growing. The biggest thing I don't care for with it is there's no way to efficiently categorize photos beyond making physical "albums". The database stores info about each album and photo but the physical storage is still folder based. It has keywords and they are searchable, but there's no way to build out the site so the viewer can switch around how they're viewing it really -- like by camera or lens or artist or macro... You can set up additional (sub)albums and "copy" photos into multiple places but that is just clunky for me.
I've been looking at
SlideShow Pro and will be giving it a try out (I need to upgrade my version of PHP past 4.3.10 first) as I am also wanting to go with a flash or javascript based gallery solution. I seriously considered developing my own but I just don't have the time it requires for that kind of personal project right now.