Highly recommended

The plugin is very easy to use, and the fact that you can set up experiments through the WP dashboard is a big plus.

By far the best thing, though, and the reason I would really recommend Nelio, is the support. The plugin is being aggressively developed, with new features being added all the time. Also, if you have any problems, they are really, really helpful. When I had problems with the plugin at the beginning (due to a plugin conflict, as it turned out), they spent the best part of a day on my site finding the problem, and then released an updated version of the plugin the same day!