I'm a newbie to the Harpy, and hawkbills in particular myself, the Harpy is a *great* knife, solidly built, *NO* horizontal or vertical blade motion when open, locks up as solid as a bank vault, it feels like a fixed blade knife when open
i'm no expert on using a knife defensively, but i'd reccomend not looking at the knife as a self defense "weapon", it's a tool, great for opening boxes, cutting fruit (oranges and apples), opening envelopes, cutting ropes in a marine environment, etc...
a knife should only be used as a defensive tool as an absolute last resort, after situational awareness, avoidance, running away, pepper spray, etc... have been exhausted, if you're in an unsafe situation and you flash the knife *first*, it might end the situation, more likely, it may *escalate* it...., and if you strike first, *you* may be considered the assailant, not the perp.
a knife is a defensive tool of last resort, you'll find most of us here on Bladeforums actually dissuade/downplay usage of knives as defensive devices
hawkbills are incredibly good user blades for day to day chores, the Harpy is good for heavier uses like breaking down boxes and such, for light use like opening envelopes and cutting packing tape closures on packages, it's a bit of overkill, the Spyderco Cricket is a much better choice for the little stuff like envelopes and packing tape, and opening hermetically sealed clamshell packages
go for the Harpy, great knife, but be warned, you will come down with a disease called "Hawkbillitis", the Harpy's a "gateway" knife, once you find it's many uses, you'll start wanting more hawkbills, like the Tasman Salt, the Cricket, the Dodo, the Civillian........
