I've done quite a bit of work with this plugin - to the point where I built about 5 different sites to push it, break it and use it hard.
It's simply amazing. It works and really does allow WP to be a CMS.
Usage & UI: Excellent
Power: Awesome (still want custom tax types!)
The best part though? You *can* build 100% custom content types with it and create multiple relations and the like. You can ALSO use it just to extend post types registered by other plugins - so easy additional fields and relationships for products, news items and the like.