Based on my experience as a developer and user I can't understand how PayPal got so big and ubiquitous.
I had the worst experience ever in trying to set up our recurring billing system with them, wasting several days of work, to the point that I simply gave up, switched to Stripe, and got everything up and running, in production, in hours.
Even as a consumer I had a terrible experience when trying to send money abroad, for my rent, with Xoom, a PayPal service. They initially took the money from my PayPal account, told me that the money was on its way to the recipient, only to block it some days later for non specified reasons (refunded in my account). On top of that, I lost the option to use my PayPal account to send money, for what appeared to be a technical issue.
Even if the system appeared to give me the option to use my PayPal account as a source of funds, telling me that the accounts were indeed connected, then when actually sending the money the option disappeared.
To solve the issue I called Xoom, they told me that there was no way of reconnecting my account and that I had to contact PayPal. So I did and they told me that they couldn't do anything and that I had to contact Xoom. After few exchanges back and forth like these, I simply asked to close my Xoom account so I could open a new one and restore the link between my Xoom and PayPal accounts, but they told me that that was not possible and that to cancel Xoom I had to cancel also PayPal.
So I gave up, used another service and got my money to my recipient.