I use the sticks and a torque wrench because I do not do this professionally. I only work on my cars for maintenance and repairs, so sometimes I go a few months without using any tools. So I do not have the habit or the feel for using the impact and the torque sticks give me the safety of not going over (or not too much - which I have definitely done in the past). And because they are my cars in my driveway I take the time to use the torque wrench to double check.
I have only used them on wheel nuts so far and all my cars have what I would call a wide range for accepted torque. I think the fact that they are all the same (or very close) is more important than the value itself (within the range)... and the torque wrench gives me that.
Now my scientific mind (scientist but not in the mechanical field) is curious... given a tight bolt, is there a way to know how much torque was applied? I know the snap-on tq will give you the final torque after turning a bolt x degrees... but what about after the fact?
You hit the nail on the head. The fact that the lug nuts are close in torque with respect to each other is more important than the torque value (of course within the torque spec plus minus limits). If the spec is 100 ft lbs, I'd rather have them all at 95 ft lbs then they be all over the place. If one is way low, it could loosen then the load that was divided among 5 lugs is now divided on 4 lugs. The remaining 4 could be carrying more than they were designed for. It can go downhill from there......
There is a simple way to test and see how much torque was applied. Take a wheel and torque the lug nuts with the torque stick/impact method, then use a digital torque wrench to measure the torque required to remove the lug nuts. (The digital torque wrench has a mode to record the peak torque.) Then take the same wheel and torque the lug nuts with the torque wrench. Then do the same thing and measure the torque required to remove the lug nuts.
To understand the essence of this requires understanding of threads, how an impact works and strength of materials and how they relate.
Threads
Of the simple machines, threads are basically an inclined plane, just wrapped around a cylinder. After the fastener is snug, the fastener stretches as it continues to be tightened. You obviously can't see the stretch by eye, but it is this stretch that provides the clamp load to hold the mating parts together. The torque is an indirect measure of clamp load. We use torque as it is much easier to measure than fastener stretch. So clamp load is what we're after using a torque value as the indicator.
Impact Tool
The impact as the name implies, does not apply constant torque. The tool periodically attempts to tighten the fastener, not constantly. There are some good YouTube videos that show how.
Strength of Materials
The torque sticks are designed such that they torsionally give when the torque exceeds it's rating. So as the gun 'impacts' the stick is really twisting at that point, thereby absorbing the excess energy. The two factors in the design to control the 'give' are geometry, diameter in this case, and the strength of the material. If the same material is used to make a set of torques ticks, then they will have different diameters. If a set of sticks have the same diameters, then they will be made of different strength materials.
After a period of repeated use, there comes a point the material will fatigue and the sticks are no longer as accurate. Maybe. In theory they would be designed for infinite life taking the fatigue of repeated stress into account. The other thing this highlights is quality control of the material.
I'm a mechanical engineer, I use torque sticks, and I have my wife and kid in the family car. No issues. And to top it off......mine are from HF. I also did a check before I used them.
I would also say if I was in business, I'd keep a data log, and do the test once every 6 months on my sticks to ensure they are operating correctly if the manufacturer didn't recommend something.
Now notice the sticks are usually painted......I'll stop there, that's another topic as to why.