An excellent plugin, very well coded and documented. WP beginners will probably need some help actually doing things with the data in their custom fields, but this interface is very slick.
The developer's free and premium add-ons are of a very high quality, I'd highly recommend wp devs buy the 'options', 'repeater' or 'gallery' add-ons, its $25.00ea for a lifetime of unlimited use including redistribution in commercial works (ie you can sell a theme with this plugin and the add-on included)
If you're new to WP, perhaps check out the plugin 'types' as well, as it has similar functionality, and allows you to add custom post types and taxonomies. If you just need fields however this plugin offers a slicker UI and much more extensive and well documented API than types.