Like most things in the world, I think it comes down to personal preference. The wider the fretboard is and the harder the strings are, the more the player will benefit from the radius. Try playing a bass without plenty of radius and you will know what I mean. Also, if a player has small hands, or is older and has a harder time hitting certain chords, radius will help.
We recently started adding a 12" radius to our fretboards and we have had a very positive response from our customers. I think that one benefit of the radius is that it allows the builder to set up the ukulele better. Since switching over to a radius fretboard we have noticed that we rarely have fret buzz issues. I am not sure why exactly, but it is nice. This has allowed us to set our string action lower without running into problems. Almost every customer comments about how much like like the setup job.
Check out the comments on our webpage. Several customers mention how much they enjoy the radius and the setup of our ukuleles:
http://www.cocoboloukuleles.com/index.php/2014-05-10-08-22-16/happy-customers