I drive a late (September)
I drive a late (September) 2013 that has HOLD mode. I use HOLD a lot. Be careful though, If the battery is kept above 50% you run the risk of overcharging it while going down hills.
Mountain mode is used to be sure the battery has 17 to 20 miles before you drive up a mountain. The GENERATOR by itself cannot add enough charge to the battery on long uphill grades on mountain roads. The battery will naturally decrease charge while driving uphill. The battery may go below 5 miles while going up long steep roads even with the generator running. As soon as you get to a flat area Mountain Mode charges the battery up to 17 to 20 miles again. The engine will stop at that point and when you go down a long hill regen can charge the battery to 33 to 45 miles or more. Mountain mode (MM) stays off until you reach 17 battery miles then turns the generator back on. The range meter can read over 50 miles downhill on regen but none of this counts on your car electricity calculator. Hold mode works the same way. It only charges to the point where you turn it on. It can hold the charge at 33 miles or at 5 miles forever but will increase or decrease according to the road grade.
Now for the facts... If the battery is depleted and you turn MM on - the car will get about 20 mpg for 20 minutes on a flat 65 mph road or zero while parked (charging is like it is plugged into a 440 fast charger). You get faster charging (15 minutes) while parked but no driving progress. After that, continuing MM on a long trip will get about 39 mpg on flat roads. Hold mode will get the same mpg results on a long trip. AC will decrease mpg by one mpg, or 38 mpg. Around town is always at a lower mpg.
Hot weather, with AC, on 2000 mile trips, with gas only - gets me 38 mpg. Cold weather 1500 miles (32 degrees and electric heater) got me 28 mpg. Note that I never charge the car on long trips. I use gasoline and MM only to keep the battery at about 17 miles. In cold weather it may only read 10 miles. I use HOLD when I don't want to engage MM. Hold is best on long flat road trips of 200-300 miles.
One other benefit in hot weather of the VOLT and MM, If you pull into a rest stop with 17 miles of electricity and it is HOT outside you can crack a window, turn the engine off and run the air conditioner for three or more hours if you want to take a nap. KOOL!!!