Comfortable enough but slides
I found this pillow comfortable.  I like to lay in the tub, Because of my back, as well as not having enough at the base of the tub to lay back my head (it would be up against the wall) I use this pillow on the side and put my head on the side of the tub and lean by shoulder against that wall of the tub that faces out.  It is smooth but the suction cups do not hold very well.  I know this is peculiar rating, as I use the pillow differently than most.  Still, it is better than the rolled up towels I used to use, so I will give it 4 stars.
Rating: 4 stars