Good question and one that I hear a lot from my own customers. There are a few companies out there that shall remain nameless in this thread that supply and fit damp proofing bricks.
The idea may make a lot of sense at face value but you do need to look deeper and understand the real concept behind rising damp, the physics and the chemistry in order make your mind up.
Rising dampness is ground water rising up the structural masonry of a building by what we call capillary action. Basically absorption of ground water by the bricks.
Contrary to belief, ground water is not actually fresh/pure water. It contains salts such as nitrates and chlorides that it picks up from the soil.
Now, If you can imagine for example that each litre of ground water contains x amount of ground salts. This ground water rising up through the masonry bring these soluble salts with it will start to evaporate from the wall surface. As you know, salts do not evaporate with the water and are left within that masonry. As more salt rich water is absorbed into and evaporated off the wall, the salt concentration gets higher and higher.
Now, these salts are what we call hygroscopic. They will actively absorb moisture from the surrounding air. If they are found on the surface of a wall, the moisture they attract from the air will dampen that wall. This is called salt dampness. Even if you removed the ground water source for the wall but the salts remained, the salts would continue to cause dampness to that wall by absorbing moisture from the surrounding air.
Evaporation rates are extremely variable. Firstly you have to look at the surface area of the substrate you are testing, followed by the ambient relative humidity of the surrounding air. The drier and warmer the air, the higher the evaporation rate.
It's safe to assume that if the evaporation rate was higher than the ingress rate, the wall would be relatively dry. If the contrary was true where the ingress amount was higher than the evaporation rate, the wall would be perpetually wet. Makes sense right?
The way these damp proofing bricks are claimed to work is that you are providing a higher evaporation rate than the ingress rate by increasing the surface evaporation area by drilling holes halfway into the wall and fitting a vent into them.
My questions would be;
1. How can you guarantee that the evaporation rate with the vents is going to be greater than the absorption rate of the brickwork from ground water and what on-site tests do you carry out that can prove this? Because environmental and ground water levels are constantly fluctuating.
2. How is this system going to combat existing hygroscopic ground salts that are found in the masonry and surface plasterwork and will still be entering into the masonry and surface plasterwork?
There are further questions that I have personally asked to companies such as these which have not been answered rationally when they were not aware I was a damp surveyor.
Now to cut to the meat of your question. Any reasonable damp proofing company should not generally be selling a product, they sell a service. A service of a system that is proven to work against the issue at hand. I personally use a range of products that I feel best suited to that issue as I am the one guaranteeing the success.
A damp proofing product is never a 'one size fits all' solution.
My own opinion is that if I believed (and there was irrefutable evidence) that the product in question was as good as it claims to be, we would all be installing them. I would have no hesitation in singing it's praises as the miracle cure. But unfortunately to my knowledge and 27 years in the trade, this is not the case.
I like to call it homeopathy for homes.