I've been trying to employ a decent gallery plug-in for WPMU and have come up short. I was able to get NextGen Gallery to work, but it takes up a ton of screen space and is overly complicated.
I found I nice little non-flash, coverflow-like carousel called Imageflow2 at http://stofko.ca/wp-imageflow2-wordpress-plugin/ but it doesn't work with MU out of the box. After many hours of trial and error hacking (I'm not a professional programmer) I got it to work nicely on WPMU with the following few changes:
On lines 180, 270, and 363 of wp-imageflow2.php, change this line:
$plugin_url = get_option('siteurl') . "/" . PLUGINDIR . "/" . plugin_basename(dirname(__FILE__));
$plugin_url = get_option('siteurl') . "/" . PLUGINDIR;
I had to disable the groovy reflections until I can install the GD PHP library.
Overall, I'd say all the galleries I found were surprisingly over-complicated. WP seems to require that images be associated to a particular posts/pages, and most of the gallery plug-ins I found were heavy in terms of screen-space and memory requirements.
I'm concerned about providing any of these gallery tools to my non-techie clients. Maybe, a super simple Gallery plug-in is on the WPMUdev horizon?