This is an update of my earlier post.
I gave up on the SmartVent in March of 2011. I still believe it is a clever design but here in central North Carolina the outdoor dew point is simply too high for it to do much good. Now I have sealed my vents with rigid foam and caulk and I've got a cheap ($200) dehumidifier in the crawl space. It is controlling the humidity very well. I let it drain into the drain pipe for my AC so no emptying or additional plumbing required.
I believe the real issue in crawl spaces is the dew point; not the humidity. Relative humidity is very misleading. Dew point is the temperature that water condenses out of the air and the issue for mold is condensation. If the dew point under your house is higher than the temperature of the pipes, boards, etc. you'll get condensation. I had water dripping off my cold water pipes!
For the SmartVent to work for you the dew point outside should be LOWER than the dew point under your house. Check your local weather data and see what the dew point in your area can be expected to do. If it goes below 60 (I base that on my water temperature), for significant periods the SmartVent could work very well. If it goes up above 70 and stays there (as it does here) then it can't do you much good.
My dehumidifier is a DEP740EW from Edgestar. I chose it because 1) a buddy had one, 2) it had good reviews, and 3) it was cheap. I've had it since about March and it is working fine so far. I put a Kill-A-Watt power meter on it so that I can track the power consumption. It averages between 200 and 250 watts extracting about 2 gallons per day. At our electric rates its costing me about $14 per month to operate it.
I like that the dehumidifier moves the problem from the nagging thing I should do something about to the economic domain. Money tends to get my attention and I can easily measure the value of improvements I've made.
This fall I plan to improve the coverage of plastic on the ground and eliminate one place where rain water occasionally gets in.