The concept is simple: surround plants with a wall of water that collects sun energy during the day and releases it at night, protecting the plant from cold temperatures. It's like a mini greenhouse with extra insulation.
Wall O'Water advertises that you can plant six to eight weeks early, without fear of freezing. Burpee suggests using Aqui-Shield up to one month early. As efficient a system as it is, I feel more comfortable with the one-month guideline.
You begin by planting your plant normally. Tomatoes prefer a soil temperature of at least 60F degrees. Below 50F degrees and roots won't grow. If your soil hasn't warmed sufficiently, cover the bed with plastic for a few days to help raise the soil temperature.
You still need to water the plant too, though it shouldn't be as often as other unprotected plants. The plastic teepee will help keep the moisture levels of air and soil higher. Don't assume the plant is doing okay with this protection. Check the condition of the plant and of the soil moisture regularly.
With the plastic system off and out of the way, drain the water out and lay it in a place where it can dry out completely. You'll be able to reuse it for years, but you don't want to put it in storage if there is still water in it; mold will definitely develop. If you are impatient and it's not drying quick enough, you can try using a blow dryer on the cool setting to blow air into the tubes.
These season extenders definitely work. In conjunction with a mini-greenhouse hoop system like I showed in my blog, "Extending Your Growing Season with Mini Greenhouses", I think you could plant six to eight weeks early as long as the soil temperature is warm enough when you first plant. Especially in very cold regions with short growing seasons, these extenders will allow you to grow plants you might not be able to grow otherwise.
At a cost of between four and five dollars each, they're affordable and considering that they can be reused for years, you're investing an extra dollar or so on each plant. The extended season that you gain should more than repay that investment with additional produce.