We spend our HSA. For a few years, we tried to save & invest it, then save the receipts, but when I finally had to dig into the HSA to scrape together a down payment, holy shit that was a a pain in the arse. Three years worth of receipts to sort through. We are a little unusual in that our out-of-pocket medical expenses are higher than normal ($6000-10,000/year).
Now we just pay medical bills out of the HSA, and the HSA itself keeps track of our medical payments, which is way easier than having go back through years or decades worth of receipts when you want to dip into the HSA.
One other factor in my decision to spend, rather than invest, the HSA: We are unable to save enough to fund a 401k ($27,000 including match), 2 IRAs ($12,000) and an HSA ($7000) every year. So basically what we do is like taking money tax-free from the HSA and using it to further reduce our current taxable income in an IRA. We realize all of the benefits today, rather than waiting into the distant future, and it reduces our paperwork. Win-win.