If you know what you're getting this is a great lens and a decent value—if you're already got the 28mm. It is decently sharp but there are noticeable aberrations off center. You can get better IQ from a dedicated fisheye lens but there aren't many AF options and most are designed for SLRs so they are even heavy and bigger (and this combo isn't svelte!).
The auto-EXIF data for stabilization and post is useful too (the camera knows when you've got it attached and registers the combo as a 16mm f/3.5 lens).