I'm pretty much 100% in a total bond market fund. I actually think the returns will solidly pay my typical medical expenses for the whole year, and then some.
Granted, a common strategy does involve paying out of pocket, saving the receipts, and letting the money in the HSA grow. There's only so much you can contribute, so it's smart to let it grow. I'm not a terribly great optimizer, though, but I do like having a huge chunk of money laying around in non-taxable bonds, earning me some money. YMMV.
(this is in stark contrast to some of my other accounts, which are pretty much 100% VTSAX, but then I also intend to use the HSA for medical expenses. There are better strategies out there, though)