Uber and Lyft have taken the world by storm.

Business Insider spoke with more than 20 drivers to learn more about what it's like to work for the ride-hailing apps.

Topping the list of complaints was smelly food and crumbs, but other pet peeves might surprise some riders.

Every day, an army of nearly 4 million drivers provide 16 million rides and deliveries across the world for Uber, according to numbers the company provided ahead of its massive initial public offering.

Those drivers meet all sorts of passengers, good and bad.

In recent months, Business Insider has spoken with more than 20 drivers for Uber and Lyft (as well as other ride-hailing apps, including Via) to learn about their experience working on the platforms.

Together, these verified drivers have provided more than 10,000 rides on the Uber and Lyft apps. Business Insider is publishing only their first names and general work area for their privacy.

Every driver we spoke with said that they've had problems with rude or smelly passengers and that tips are often few and far between, even with the addition of a tip function in the Uber app about two years ago. Still, they described a workplace that's highly flexible around a driver's busy schedule, and instant payments are a big plus for those who need the money quickly.

Have you driven for Uber? Have a story to share? Fill out this form and a reporter will be in touch. You can reach the author of this story at grapier@businessinsider.com.

Here are drivers' biggest pet peeves when it comes to the passengers in their back seat.