Inspired by a post by @xplosive I started digging into HBD a little to figure out what to with mine, whether to stack them, put them into savings or flip them into HIVE, LEO, SURGE or maybe BTC.
This is the first real post I'm writing, and it's mostly for me to summarize what I know and hopefully get some feedback.
I have my fair share of experience with decentralized stable coins, e.g. I held a few thousand UST when LUNA crashed, so HBD is very interesting, especially seeing it work for such an extended period of time.
So HBD retains a soft peg against USD because theres (basically unlimited) liquidity against HIVE, you can swap your HIVE into HBD and back, it comes with some waiting, a bit of process and (when swapping HIVE into HBD) a 5% fee. As you always get 1 HBD for every USD worth of HIVE, there's basically an arbitrage opportunity if case the price of HBD exceeds 1 USD. Less the fee, it means it's acceptable for HBD to trade up to $1.05. Okay, I get that.
The other way isn't so obvious: when HBD goes below 1 USD, then there's an arbitrage opportunity to buy HBD, swap them into 1 USD worth of HIVE and sell the HIVE for 1 USD. Sure. But where do these HIVE come from? Do these get newly minted? (I know I didn't ask where the HIVE go when swapped into HBD).
In until here, it's possible a major depeg of HBD could cause a lot of sell pressure on HIVE, but that's where the waiting times and, even more so, the haircut rule are coming into play. The total amount of HBD isn't supposed to exceed 30% of the mcap of HIVE, and if it does, you'd get less HIVE per HBD. This way, an issue with HBD can't really spread over to HIVE. Smart.
(I wish the LUNA guys had looked at HBD)
So I guess that the main question for me is where the 15% APR are coming from. If this is just inflation of newly minted HBD (which I suppose it is) then it means this will continue to create sell pressure on HIVE if folks want to liquidate their interest, and seeing that the main way of cashing out is through HIVE, given the limited liquidity for HBD on exchanges.
It overall seems a very solid model. It also means if there's a lot of demand for HBD, this will drag the price of HIVE up long-term. If I ever get to stack a bigger pile of HBD, I might actually put them into savings because 15% on stables, if sustainable, is extremely attractive. I'll even consider flipping some of my other stables into HBD.
It also means there's merit (imho) in fostering adoption of HBD in general, as there's been a debate in terms of the value of @SpendHBD etc, so that's great! 
I hope I summarized this correctly, and that if not, someone will come and tear this apart :-)