I’ve been using the superfast Shutter Reloaded for my images and i’ve never changed it because of it incomparable speed; yet, its super-minimal interface – which i personally liked a lot – was sometimes confusing for users. So i had to look for an alternative to be both fast to load, pleasant to see and intuitive. I’ve probably found it: jQuery Colorbox
JQuery Colorbox is a fully configurable wordpress plugin that makes use of Colorbox to show images. It has a lot of options but to start working it only needs a click on the first one. It let you choose the style of the overlay between 11 themes that fits both dark and light sites, at works out-of-the-box with WordPress Galleries and NextGEN Gallery!
I am testing it and using it right away: see it in action on this site!
Plugin page on wordpress: http://wordpress.org/extend/plugins/jquery-colorbox/