I know 2 people who have bought the current gen Prius - one regular, one plug-in.
In both cases, the local Toyota salesman told them: "Use B-mode on the shifter to save even more gas when coasting! It increases regen."
Bad advice!
Yes, B-mode it increases regen, but not to any level that can't be achieved simply by pressing the brake pedal lightly.
Worse, in the regular Prius, B-mode also spins up the engine (without adding fuel) for extra drag. So if saving fuel is your goal, you're in fact wasting energy by using B-mode and converting kinetic energy to heat (ICE friction), compared to just using the brake pedal.
PRIUS PRIME B-mode
I wasn't 100% sure if the Prime behaved in the same way, so I tested it this week with the ScanGauge.
Interestingly, B-mode behaves differently in EV (charge-depleting) mode than it does in normal hybrid mode. According to the ScanGauge, it will spin up the engine in hybrid mode, but not in EV mode. (Tested only at city speeds.)
My Prime friends had been using the car almost exclusively in hybrid mode, and also using B-mode often when decelerating. This new info will help them save a tiny amount of gas.