Just used it for the first time 
(successfully at least… I was the first Monzo user to have a failed PayPoint topup 
- a couple of weeks ago)
Anyway 
The Monzo-side experience was perfect, notification, 10 minute wait, money in the account available to do with what I please 
The PayPoint-side of things was less seamless… 
(nothing I couldn’t cope with though 
)
I handed my card over, there was a triple take… “Mastercard?!”, “Monzo?”, “TopUp?” - a bunch of confusion, 3 failed attempts (as the cashier was inserting my card instead of swiping it - very important).
So I asked them to take a photo of the error message, I was prepared to disappointed that it hadn’t been enabled across all of PayPoint’s network but then I realised the cashier was inserting my card into the machine, I asked them to swipe it (for attempt number 4 
) and tada! It worked
So just a word of advice, make sure that the person operating the PP machine swipes your card and doesn’t insert it 
Perhaps that should be added to the Add Money screen @nickrw?