Kroil does an absolutely outstanding job breaking loose rusty fasteners. It has a stronger odor than some people prefer, though not nearly as strong as the WWI chemical warfare assault perpetrated by Montana Extreme's wares. Kroil probably does a decent job with some types of fouling, but there are stronger, albeit more expensive, surfactants available for bore cleaning.
I love the stuff. It's all the best cancer-causing agents built right in so you don't have to look for other carcinogens to add to it I do like it but if you are wise you'll use gloves & plenty of ventilation when using it as a penetrant. There are safer and better options out now. Castle Products makes a penetrant called Thrust that foams and stays in place better and works as well or better than Kroil. Safer too. Neither are designed to be lubricants as much as they are penetrants/cleaner. Thrust can be found at many NAPA stores. Being in the auto repair business for 25yrs I have found that the Thrust product is unsurpassed as a penetrant.
Maybe the better question is what are you looking to use it for? Kroil is designed for certain purposes and is a good product when used for those purposes but most of its uses don’t really fit well into gun maintenance. Do you have frozen parts or are you contemplating use as a lubricant? As a lubricant, there are better alternatives that were specifically designed for firearms. As for frozen parts, it is best to determine the exact cause before pumping chemicals at them.
If you use it to remove gunk and buildup in hard to reach places you would want to be able to purge it afterwards. It does have a caustic nature about it and I would refrain from using it around wood, plastic and composite parts. It can get nasty on non metal finishes. I have it at home but never use it on my firearms; got plenty of other, better chemicals for them!