There is now new 4 card slim wallet version 2 and a 6 card wallet. The new designs fit nicer in the pocket and have improved finger reliefs to get cards out.
I had a leather slim wallet that had a similar design but was falling apart. This wallet has been far better, cheaper and should last much longer. Also looks pretty cool compared to the average leather wallet.
Update: I have further improved both wallets. I have made the 4 card wallet a bit taller and added a notch to help taking money out easier. I also made the finger cut out more effective. Same notches have been implemented in the 6 card wallet as well. There is a double notch and single notch version for each. The double notch is to help get a card out of the money slot but it is still difficult to do so.
NOTE: I have added two small supports in the money slot to help keep the walls in place. These are easy to remove with a utility knife or small flush cutters.
The bottom tends to curl a bit so a clean hot bed is very important and it is a very good idea to use a brim and trim it off with a knife after printing.