As a marathon runner this stuff is a must-have. Anywhere you have skin on skin or clothing on skin contact is susceptible to chafing - typically areas under your underarms, upper inner thigh, inner bicep, etc. For me I can't go more than about 6 miles in loose running shorts before the fabric causes chafing on my thighs.
I don't use this when running in compression shorts or tights, of course.
Vaseline is a cheap fix but it stains and doesn't last very long. Aquaphor lasts a little longer. This stuff has a waxy component that makes it stick around. DON'T USE TOO MUCH! You don't need as much as you might think, if you use too much you will stain your clothes (looks like an oil stain) and it's not cheap.
I also use it in the appropriate places when I do long distance cycling. Also good for your feet and hands to prevent blisters.
Plus:
Lasts all day
Handy applicator
Less less than alternatives
Minus:
Pricey
Faint smell (not a bad smell, but noticable)
Some minor staining if you use too much
If you have questions or comments, or found this review helpful, please let me know!