This is useful sure, at least now you can save a Bitcoin address for a contact and not have to reuse the address which is even worse for privacy, but let's be honest, it's not private at all.

The sender can very easily find out the real address the btc was sent to simply by monitoring the timeframe in question for the exact amount he knows he sent.

Anyone else at a later date can do the same.

I would argue this is more of a convenience feature than anything to do with privacy. 

 The sender can just inspect his transaction history if the wallet is advanced enough. This just makes it easier to avoid address re-use. 

 This makes it impossible for anyone other than the sender to see any payments to the silent payment address. So you and I could both pay the same publicly posted silent payment address and we would never know about the other payment, and it would not be possible to look it up on a block explorer either. 

 The sender can find out the real address on send, because they are the ones who create the output. But they can't see any other sends to the silent payment address from any other sender. And no, nobody else can figure out any other payments to the recipient without the recipient's private key.