At the end of the day, it really depends on what the purpose of the additive is and what you're using it for. I've had good luck using Liqui-Moly additives, and they all worked well for their purpose. Examples:
Motor Oil Saver
When I bought my Grand Marquis, the valve seals were not exactly doing their job. It's a huge job to replace valve seals in this application, and I decided to save it to when I upgrade my camshafts. The valve cover gaskets were also leaking.
When I changed the oil, I added a 300 ml can and within 900 km, there were no more puffs of blue smoke when accelerating from a standstill. It also slowed down the valve cover gasket leak, which was a definite improvement compared to what it used to be. I made a habit of adding a can at every oil change, and over time, it even addressed out an issue I wasn't aware of - weeping o-rings on the oil cooler hose. Using it gradually, now the valve covers barely weep.
It's composition is nothing magical (2-butoxyethanol and dioctyl sebacate), and it does not claim to do anything else. It's done its job perfectly and I would highly suggest it to anyone with oil leak issue. That said, its sealing effect sealing effect depends entirely on the gasket material.
Cera Tec
I've been using MoS2 for quite a while, until I was recommended to switch to Cera Tec by the R&D Manager. Instead of using it every 30,000 km as recommended for severe service (they recommend 50,000 km for regular), I was suggested to use it at every oil change for extreme duty. I haven't really noticed much of a difference compared to MoS2, but my engine used to consume about 250 ml of oil between oil changes (10,000 km). I'm now 9,400 km into my interval, and there hasn't been a noticible change in the oil level.
ATF Additive
I've had a power steering leak, and didn't really have the time to pull the pump off and rebuild it. Instead, I added some of this stuff, and the leak disappeared a week later. The fluid also changed color, so I decided I would replace it. I added some of this stuff afterwards as preventive maintenance, there was no color change and I reckon the cleaning additives were doing their job the first time round.
Pro Line Engine Flush
Many claim using flushing products are pointless, because all the cleaning agents are already in the oil. An engine oil does many jobs, and it won't really be realistic to expect 100% for every function. Using PLEF at every oil change interval, the engine started to idle and rev smoother, there's been a drop in oil consumption (1 liter every 8,000 km to 250 ml every 10,000 km) and compression is still 200+ PSI on all cylinders.
Provided you use a quality additive for exactly what it is intended for, I do not see a problem using them. But your expectations have also got to be realistic.