If I lived in one of those cities I don't know if I would be excited for the potential city prosperity or scared to death than the city would become unlivable due to increased traffic, increased property values (taxes), or b/c my city/state gov't gave away so many tax credits.
I think, for my family, having Amazon build their HQ2 here would push us off the fence of gradual QOL decline (gridlock, inadequate infrastructure, high costs, etc.) and either catapult our comfort level up through increased prosperity, or push us out of the city because we haven't seen enough wealth increase to outpace the additional costs and scarcity HQ2 has brought.
I'm pretty sure I'll never find out, though, as the chances of Amazon picking Toronto seem extremely low. At least the slide into unlivability is predictable.