I'm going to answer or at least provide information from a different view.
I love warm boots. Though many folks will tell you I spent my money willingly on ski gear, I always look for good deals or an alternative. For warm boots, I've gone the alternative route.
When it comes to ski boot bags, I love the Swix line. The one I have is the Swix Norwegian Team bag. I found them better than Transpack and it has been very infrequent that I couldn't fit everything in it. Along with what normal people bring, I usually have three pair of goggles, a knee brace, thermos, water bottle (camelback type), video camera, helmet, 2 pair gloves, mittens, portable hair dryer, alpine tech manual, drill cards (thanks Rick-Your Ski Coach) and all sorts of odds and ends in the side pockets.
I will admit that when I was in a ski shop this past season, I saw the new pro line Transpacks and thought they were well made (more durable material) and more spacious than previous models, which I have owned.
My route for heating my boots is slightly different than most. I usually have a 35 minute drive to where I coach. If it is just normal east coast cold, I use the little orange Dry Guy heaters. I might slip them in the boots while still in the house getting ready and start warming them and then plug them in the car outlet for the ride to the mountain. They come with a AC/DC charger. In the house or in the car, the boots are in the boot bag already. When I get to the mountain, I slip my foot into a nice warm boot
If it is wicked cold (curse your momma for having you type), I take the boots out of the bag and put them in the floor of the car and let the car heater work its magic. I might even have the dry guy things inside the boots as the car heater will only heat the outside. You can try angling the boots so the hot air flows down the cuff, but the first frost heave will knock that out of whack so I don't rely on it. When I get to the mountain I put the boots in the boot bag, hike inside and put on toasty warm boots.
I know all that might sound cumbersome but it really isn't. My dry guy approach is almost identical to the Heated bag one except it heats from the inside out instead of the outside in. My floorboard approach is the same as the heated bag (outside in) but you have to put the boots back in the bag. I usually travel solo so there isn't an issue there, but even when my daughter and I have traveled together, she hasn't minded having two pir of boots on the floor boar under her feet.
On the wicked cold days I often will throw the Orange dry guy things in my boots at lunch time to warm them up again. I also have the hairdryer to do that (I use it for drying/warming the liners and not so I can get in and out).
I have also warmed the boots in the house using the dry guy boot/glove heater (not the little orange things but the inside table top model) before putting the boots in the bag. That works well also. I just heat them up for 20 ish minutes, put them in the bag and then head to the mountain. Since my drive isn't long, I still have nice warm boots. On a longer drive, I could still do one of the other approaches too.
So, all that and I always have warm boots and I didn't spend big bucks on a heated bag that sounds like it has reliability issues.