If you ski every day a boot dryer is a must. Even if your ski boot is totally waterproof, the inside will get damp from foot sweat alone. If you do not ski everyday, but some consecutive days, a boot dryer is needed as it typically take at least 24 hours to dry out a boot. Recently I was asked my thoughts on boot dryers. Having owned a few; here they are.
Upside Down Dryers-USD dryers are typically what people think to be the best dryers. These blow hot or warm air into your boot. Some have fancy timers and additional slots for gloves. The Dry Guy Wide dryer is one of the most popular models. Four drying slots allow for both gloves and boots to be dried at the same time. An optional boot extender is available too. This dryer only gets pulled out when we have a bunch of guest and it has been really wet out. For personal use, this unit is a bit much. But nice. 
Another popular USD dryer is what I call the "outdoors man" dryer. These come in many colors and brands, but are all of the same construction. Found at outdoors outfitters such as Cabelas and Northern Tool, these dryers have been around for a while. I have even seen them being sold at the local drug store in the winter. I have heard many good things about these dryers but do not have any first hand experience.
Though USD, do the job well, they are not my preferd dryer. One reason is that warm moist air rises. In the USD position, forced air is required to dry the boot. If I am drying my boots overnight, I typically give them a blast of warm air and let nature do the rest. The next reason is that ski boots are heavy. Getting the boots to stay on the dryer often requires a Jenga winning balancing act. And fianlly, USD dryers take up floor space. Unless you have a dedicated spot for your dryer, this is a appliance that will need to be put away when not in use; the smaller the better and these things are not always small.
In Boot Dryers-These little gems are the foundation of the boot drying world. From small electric resistance heating units to simple forced air units, these dryers do their job and take up little space. Boot heaters/dryers work simply by heating up the wet boot and allowing the moist air to escape out the top. These dryers will take overnight to dry one pair of boots. They are cheap, durable and take up little room in your ski bag. These are perfect for traveling ski vacations where consecutive ski days will naturally occur.
I would not use this unit for daily home use as I think it is waste a decent amount of electricity. A timer can be used so that heating/drying occurs only half the night for less wet boots.
Saving the best for last are the forced air in the boot dryer. The two piece version I have is no longer available but are essentially the same as the pictured unit. Heated air is forced into the boot and circulated out. The key to this type of dryer is the tube which directs the air all the way to the tip of the boot, or at least down into the boot a way. Without such a tube, the air would not properly circulated. This type of dryer also has extra drying ports for gloves. Though the one I have uses spent air from the boot drying process to dryer the gloves. This sometimes results in smelly hands. With the two unit version I am able to leave my ski boots in the boot bag and give them a quick shot of warm air after my ski day (On non-consecutive days of skiing I do not dry out my boot.
Regardless of which device you use or how many days you ski; dry boots at the beginning of the day and wet boots at the end of the day typically makes for a GOOD day!
