Although the payout is not instantly I dont deem the payout scheme to be worse than PROP payout as soon as a block is cleared and credited.
We save all the valid PPS shares (along with a timestamp) in a queue and credit them one after another to the user as soon as the pool balance can allow this. Basically this means every time a block is cleared and credited, the next time when the queue will be checked, it will credit PPS shares to users until there is no more balance left. So it might happen a couple of times, that you cannot cash out your rewards instantly, but on the other hand it can also quite often be that you can cash out your shares before a block has been cleared.
That is not proper PPS though. PPS is supposed to be risk-free for the miner, but from what you describe, the miner is risking not getting paid for ages (or ever) if your pool gets long rounds.We save all the valid PPS shares (along with a timestamp) in a queue and credit them one after another to the user as soon as the pool balance can allow this. Basically this means every time a block is cleared and credited, the next time when the queue will be checked, it will credit PPS shares to users until there is no more balance left. So it might happen a couple of times, that you cannot cash out your rewards instantly, but on the other hand it can also quite often be that you can cash out your shares before a block has been cleared.
We at rfcpool.com used Prop at first and then added PPS too. It started out tied to found blocks, but that was rather unfair so we soon fixed it, so that all PPS shares went instantly to the confirmed balance that could be paid out at any time. But the rounds were long and the pool was incurring losses. The owner paid the miners out of his own wallet, but eventually got tired of the losses and shut it down.
But since we had written the pool website from scratch and it was full of beautiful statistics and stuff, and the servers were robust enough to handle spikes well beyong 200 GH/s (no closing down of registrations for us
) - and the users that had to move elsewhere were complaining that the other pools' websites suck and they're unreliable and unsustainable - the owner got strong-armed into reopening 
So now we're using PPLNS, because it won't bankrupt the pool, it's fair to regular miners (hopper-proof) while being easy enough to understand, and on average miners get the same rewards as on PPS. Everyone is welcome to check us out