I've had my Gear Vendors unit for several years now in my K5 Blazer. Had it hanging off the back of my NP205 transfer case when I ran a TH350 auto, and now have it hanging off the back of the same NP205, but now I run an SM465 4 speed. Probably the best upgrade I've ever made, and I've made a lot of nice performance upgrades
As others have said, it is a very strong unit. Change the oil/ATF every now and then and it will last a lifetime. As others have said, it provides nice overdrive for cruising down the freeway. But I find that I use it more to help me tow heavy loads over the mountain passes. With my manual tranny, it allows me to split every forward gear to keep the engine in the power band. I have 1st, 1st overdrive, 2nd, 2nd overdrive, 3rd, 3rd overdrive, 4th, and 4th overdrive. When I am pulling a loaded car hauler up the Cajon Pass in SoCal, 3rd overdrive is my sweets spot.
I found the Gear Vendors complete with all wiring, switches, and brain in an '86 2 wheel drive Suburban behind a TH400 at an Ecology Pick a Part several years ago when the cash for clunkers incentive was going on in California. Everyone was cashing in their older rides. Got it for $100.