I recently came across 2 posts defining the difference between usability and user experience

The post on The Battle Between Usability and User-Experience by Thomas Baekdal provides an easy to understand difference between usability and user experience using roads as an analogy.

The 9th comment on that post also provides 2 more examples on how usability alone isn't enough for a successful product. One example being the creation of the Betty Crocker cake mixes.