My personal opinion: useless.

The cost is the same in total either way, so the difference is just whether I do my budgeting or RIE does my budgeting, and I do not trust RIE to do my budgeting. Then anecdotally, seeing the high and low bills keeps me more connected to my energy use, and a high bill can be a kick in the pants to keep my energy waste in check rather than just floating along paying the same every month.

What's more, they still reserve the right to change your monthly amount if your usage or their price changes, so you're not even fully protected from unexpected bills.

I assume you're read through their FAQ, but here's the link for anyone else thinking about it: https://www.rienergy.com/RI-Home/Bill-Help/Budget-Billing