Out of the box, Plogger is certainly the cleanest-looking PHP-based photo gallery out there, but the limitation of style sheets as the sole method for customizing one's site is quite a downer. It's incredibly discomfiting to hack gallery.php to change my site's HTML structure (i.e. to add a logo image, to replace filenames with captions, etc). Now, when an upgrade is released, I'll sooner settle for my old version of Plogger than have to diff my hacked source code with a new version's.
As such, I think Plogger really needs a WordPress-style theme system for it to take off, something that separates HTML **and** CSS from the content, not just CSS.
Funny you should mention that =) You're new to the forums aren't you?
We have already completed a theme engine for the next version, complete control over the way your gallery performs and functions through CSS and XHTML. You'll never have to hack your gallery.php file again =) Coming soon.