As a skeptic, I am generally wary of any claims that say something that is generally hard is made easy by a product. I was wondering if anyone could offer some advice if they have experience with various stump removing chemicals on the market (not looking for product specific advice). Assuming that one follows the directions on the product, what is its efficacy?
Take a look at this question, Nothing will grow where tree was, here on SE.
If the stump is of a reasonable size, have you considered stumping grinding as an option?
If you go the stump grinding route, make sure you specify a minimum "grind" depth. Bear in mind, the deeper you want the stump grinded the more it may cost you.
As Mike Perry pointed out, this question on this site is a good first hand account of what happens when you use stump removal chemicals.
All that most chemicals provide is a very high amount of nitrogen. They're just glorified, over priced fertilizers and I'll bet that most products have potassium nitrate as their main ingredient. Typically several deep holes are drilled into the stump and the chemical is poured down them. Given that tree stumps will definitely rot in a high nitrogen + moisture environment, there is no question of whether these products will work or not. They will, although the process might take from anywhere from one to four years.
However, the question that needs to be asked is what is their effect on the surrounding soil. Depending on the type, trees have roots that go very deep into the ground or cover a large area at a relatively shallow depth (see this answer for some info). Since these chemicals get transported to the very tips of the roots, you'll find that over time, a large portion of your garden is unfit for growing anything due to the extremely high levels of nitrogen in the soil (will "burn" everything). The levels are high enough (and the effect extends deep enough) that normal gardening techniques like adding carbon/manure, etc to alter the ratios will not work. That ground is dead for a few years.
I know that the questions asks about the efficacy of stump removal chemicals, however, I assume the intention is to gain information on removing a stump.
A friend of mine just hired a 40lb jack hammer from Home Depot for $55 for 4 hours and said that it was the perfect tool to quickly and efficiently dig up his stump. Personally I thought this was an ingenuous idea and wanted to share it.
I know of three ways to chemically reduce a stump:
As an additional measure, drilling holes and adding soap may help water move in and out of the wood. This may help with stumps that are naturally oily, such as cedar, redwood, and larch.
None of the chemical or fungi methods are swift. Time is measured in years.
Stacking fresh manure on the stump will work as well as any of the chemical methods.
Any drilling of holes you can add to the stump will speed up decomposition substantially. They don't have to be huge. 1/2 inches, 6 inches deep, 3-5 inches apart will allow fungi sports to enter into the wood, and for manure nutrients to penetrate.
You can also burn out the stump. You need a steel barrel with one end removed. Add two tubes the closed end, about 18" long, one pointing in, one pointed out. This will work as a downdraft heater.
Other ways to deal with stumps:
Cut them off flush. Put a planter on top.
Add a slight mound of dirt. (This is what I do on my trails in my woods.