I'll say that it's just "OK". It depends on what you're trying to achieve.
If you're trying to shoot portraits and have the ring reflection in your subject's eyes, this can only be done using a Wide or UW lens up close. From a decent distance away, by using a 50+mm lens on a crop, the ring on your subjects eyes won't be noticeable. Either way, the effect isn't up to snuff IMHO.
If you want the halo around your subject, then it will do it decently, but your background needs to be dark enough, and depending on lighting, you have to go full power on the flash to do a whole body shot, or get closer to your subject. For better quality, i would suggest getting a unit that runs off of strobes like the AlienBees. Mine is attached to an SB-600, and i think a more powerful flash would would work better.
It does work well for some macro-type photography where shadows are an issue.