I got promo chips from a few different places in Vegas this year. They all worked the same. The promo (they called them non-negotiable) chips were a different color than the regular chips, and you couldn't cash them out. You bet with them as normal, and if you won they paid you with regular chips. So basically you keep betting with them until you lose them.
They are worth 100% of their face value minus the house edge of the game you are playing. Or, if you're counting, they would be worth face value plus your expected edge.