i have a customer who has set this up as a side business for his daughter and he has run the numbers for me before and it is pretty impressive.
that being said i have never heard him mention any negatives or pitfalls. pretty sure you just buy some balls at a sams and keep the machines on a route.
Profit is big in this Fish. You would need atleast 50 locations that are close to you to make great money. Its easy though. Get the right spots. Stick a sticker on it saying that part of profit goes to charity and that helps get you in the location.
i let a gumball vendor leave his balls at my business. he stops in every 2 months and takes the quarters. if you can scatted about 500 of them throughout the city it would be a nice source of supplemental income. Not sure if it would be worth it if you only had 10 or so. I would say he clears about $25 a month with the gumballs