I hate letting the air out of my tires and having to keep checking psi as I go down, so I started looking for a simpler quicker way and found a deal on these...
http://trailheaddeflators.com/warranty.htm

They are made in the USA and had some good reviews on them.
I looked at the cheap Teraflex ones, but they still need watched and checked often. I looked at the Smittybilt and a couple similar that were ok, but couldnt match the deal I found on these.
I liked the screw on and drive off, so that you can get started to the first obstacle then get out and double check they worked properly and remove them.
I got mine for under $25 shipped in a month long search and bid session on ebay. The kit I got is brand new and is worth the full price they ask, but I had the time to search and was willing to wait for a deal. Mine are the 5-20 PSI version.
Here is a write-up from Jp Magazine...
What: Trailhead Automatic Tire DeflatorUsing It: The Trailhead deflators are 100 percent made in the USA and offer a no-questions lifetime warranty-even if you found 'em in a riverbed, the company will replace 'em with no questions asked. They're available in either 15 psi to 40 psi or 5 psi to 20 psi versions. Inside the leather carrying pouch, the kit comes with four adjustable deflators, an Allen wrench, a low pressure gauge, a laminated instruction card, and a tire deflator guide. Initial setup is pretty simple. You spin the deflators to the valve stem and use the supplied Allen wrench to adjust the pressure. Tightening the screw raises the deflation pressure and loosening it lowers the deflation pressure. On our low pressure models, one full turn of the screw equates to a 1.5 psi increase or decrease. Once the deflators are set, simply screw them onto the valve stems and forget them. You can drive the whole trail with them and then take them off for airing up.Highs: Killer warranty; good quality vs. value; setup not difficult; easy to useLows: Initial setup takes a few minutesPrice: $60 (per kit)Get It: Trailhead Automatic Tire Deflators, 800/725-0478, trailheaddeflators.com
I will be hopefully setting them this weekend and get to try them out in a couple weeks. I will let y'all know how they work out.