I have a bunch of these cases - and have been a part of the conversations on these for the past few models - and I find a few things very subjective in the comments.
First - How Slippery - I don't have the largest hands, and gripping the PM phone has never been an issue. Very few cases have I ever found "slippery". With that said, I like some grip or something on the sides, but none have had that outside of the Dbrand Grip and Catalyst. As some compare them to wet bars of soap, I have never had that. The closest is when my hands are really dry in the winter air. I have used the Rebel as my everyday case for the past few years, and it has never dropped or slipped out of my hands. Same for the Mous or any other some refer to as that.
The second is the fingerprints. Unless I eat something greasy, I have had no issues with the carbon fiber cases and fingerprints. My fiancée, on the other hand - her hands are naturally more oily, and everything she touches leaves prints. If your hands are like this, you will find that most cases leave prints.
I think if you typically find your glass has a lot of residue left on it after taking drinks, you will find cases are going to have fingerprints left and will feel more slippery.