I have a custom tenor with an 18" radius. It is a pretty gentle radius and to tell the truth I conciously can't really tell the difference between it and a flat fretboard. I do like having the radius though and have a concert on order with a 16" radius. Maybe that one will be more noticable.
I'm a little biased on this because my only experience with radiused fretboards on ukes have been from some of my favorite luthiers to begin with. I have a number of ukes up on my site that have already sold but have been handcrafted with a radiused fretboard.
The photos and descriptions are still listed on my web page if you would like to take a look at some of the caliber of instruments nowadays that are beginning to incorporate this wonderful feature.