It hasn't been a complete disaster... but I would now strongly recommend against it.
- Most customers want a fixed bill for budgeting purposes. They would often rather overpay a little bit than having to set aside a special budget that they may or may not need any given year.
- It will make forecasting your revenue a real SoB. For a bigger company you may be able to annualize the payments and deal with liquidity crunches. But imagine being a smaller company and going through months at a time when you aren't getting any money in because of the cyclical nature of demand.
- Customers feel punished for using the product more rather than "getting more value" out of it.
- It's extremely punishing for your biggest customers. 10% of your customers will use 90% of your customer resources. Making them basically foot the entire bill for your company will make them not happy. You will have to find a way to cut them their own pricing or they will go to your competitor (or worse, stand up their own competitor instead of paying you).