Well, I’m definitely speaking from experience as I have a very extensive Gallery-run gallery with about 7500 photos. Gallery the application as it stands now is not something I’d like to integrate with WordPress, simply because so much of the code is not at all in sync with WordPress goals.
I had to spend hours and hours just getting all the code to approach validation, and there’s still a ton I missed. More hours were spent eliminating unecessary tables and making a CSS based layout for it. Plus it really doesn’t lend itself will to integrating into an existing site design.
Lately I’ve been running into other limitations as well. For example my new camera lets me record audio attached to pictures, or just by itself. I can’t do anything with those files in gallery. What I want is a multimedia platform that will handle my audio, video, and picture files, plus whatever I am coming up with years from now. This is the direction the multimedia functionality in WordPress is heading, something flexible enough to handle any number of formats elegantly and with clean code and accessibility in mind from the begininning.
In the meantime if you’re looking for something good Alex’s photos hack looks quite solid, though I haven’t gotten a chance to use it myself yet. Alex has also committed to providing an upgrade from his hack to whatever is eventually officially incorporated into WordPress.
