The first benefit comes when the money goes in. And of course you get that whether you spend it on HC right away or not. But to get the second benefit you need to wait until you have a bunch of tax free income in the account. But like you say, you keep the receipts and you control the timing.
The bad news for me is that it complicates the 'where to spend from' decision. I have tIRA,401k, Roth, variable annuity, after tax, and now HSA. And they all have slightly different tax consequences when the funds are pulled out. Since my after tax fraction is small, I don't have anywhere near 20 years before I need to decide where to dip in. At least it's not 2014!