I haven't tried the stuff for boots, but I have tried Kiwi Camp Dry, which is a silicone spray you can use for tents, chairs, etc. Other non fly fishing silicone liquid treatments include things like Scotch Guard and Rain X. The FF industry has similar products too, such as Watershed, and Cortland has a spray, and many others I'd expect.
It works. Flies float longer, and dry quicker after getting soaked, essentially permanently waterproofs the material. I would imagine virtually all silicone treatments are similar. But here's the skinny. The low viscosity ones like this have an alcohol base as a wetting agent, allowing more complete coverage. The silicon is bonded to the alcohol, not the fly material. While wet, it would actually sink the fly, and in fact alcohols are used as sinkants. It has to dry completely before it works. This is not a streamside thing, but you can pre-treat at the vice, or just take the spray to your dry fly box! Once it dries, then the silicon bonds to the material of the fly.
The pre-treated just make it harder to soak, and easier to dry with backcasts and so forth. It waterproofs it.
Streamside floatants, whether gels or powders, also use silicone to repel water, but they have a base which bonds to soft materials like thread, feathers, and dubbing, thus being useful immediately, and allowing you to re-apply as necessary. It might even be that the pre-treatments lessen the effectiveness of the streamside floatants by preventing that bonding.
I tend to use the gels. But pre-treating with a silicone treatment, combined with a desicant, might be the best combo.
