POC Bones here. Downsides- expensive, make you look like a knob, bulky.
Upsides- huge protection. Long-lived. Comfortable- they’re just the right shape for your arm, so rather than having to be held in place by tight straps, they really just sit there. Very, very unsweaty (they look like they should be warm but the construction means that they’re less enclosing than most, and the retaining “sock” is very thin).
Yet to find anything else that isn’t crap by comparison- though I’m tempted to get some Gforms for more minimal protection.
I got the bones to and agree with everything Northwind says about them.