How interesting it will be depends on too many things we just don't know yet (but will know soon enough):
-Is it a default passive or a secondary? If its a default passive you can combine it with other mobility items (detonator), if it's a secondary it's more limited. IMO it's more likely to be a secondary.
-Technical details about the jetpack: How do you control yourself midair? (Shield like control or normal airstrafe?) How strong is the push force? What's the cooldown on it? Can you turn it on/off/on without landing?
-To a lesser extent, the update is supposed to be a class rework, and not just a new item. Will they change some of the stock pyro mobility stats? (movement speed, weight. Think of the spy speed buff from a year ago) Also what if they change some of the mobility stats on the already existing mobility items? I personally hope this doesn't happen as it would mess [again] with various ongoing mobility projects (maps and such).
Bonus thought: how will it interact with a quick-fix medic?