Now that I've had more time to experiment with both, I can share my update.
PayPower's website doesn't work very well
In addition to a prepaid debit card (with a steep $5.95 monthly fee which, unlike the T-Mobile Visa Prepaid Card's, can't be waived – but more on that below), a permanent PayPower account also includes a rudimentary bill pay feature similar to that offered by some smaller, regional banks and credit unions. Rather than having billers pre-loaded or searchable, you need to type in your biller's payment details, which are then cross-referenced with their database to see if the payment can be made electronically or if it requires a paper check to be mailed.
The website doesn't work very well, and saved payees periodically disappear before unexpectedly reappearing days or weeks later. At other times the bill payment page completely fails to load. I suspect this is an issue with how PayPower interacts with their 3rd-party bill payment service, and it's something you should certainly be aware of.
Online bill payments are possible, however
I have been able to make online payments to my Barclaycard Arrival+ MasterCard using the payment details on my statement (the "Card Services" address in Philadelphia). Since PayPower required the same information as my credit union to add a biller, I suspect they may use the same 3rd-party service provider. I've now made two bill payments and both were received by Barclaycard in 2 business days, indicating the payments are being made electronically.
There is a substantial shutdown risk
There are many reports of shutdowns in the FlyerTalk thread on PayPower accounts and the co-branded T-Mobile Visa Prepaid card, which has all the same functionality as PayPower and is also administered by Blackhawk Network. Particular flags seem to be large online bill payments and heavy unloading activity at Walmart. This is not surprising: we want our money back as quickly as possible, while they want to hold onto our money as long as possible.
The difference is that when they don't get what they want, they take their toys and go home.
The monthly fee may be avoidable
I can't seem to find any similar reports, but by keeping the balance in my PayPower account at $0, I have not been charged a monthly fee yet. In other words, they don't charge the monthly fee against an account with insufficient funds to pay it. It's unclear to me whether my approach is sustainable or if my account will quickly be flagged for closure.
For now, my account is still active, but I plan to use it very lightly, for 2-4 bill payments per month. The ability to buy REloadit cards for $3.95 in the powerful bonus category of grocery stores is a tempting enough opportunity to keep me coming back once a week or so. I will be immediately emptying the account, however, and won't be surprised when my account is ultimately closed.
I'd love to know: what experiences with PayPower (and T-Mobile) have my readers had? See you in the comments.