I keep thinking about this - I think that unless the entire NYC market really tanks in a way that no-one expects - that JH will be fine. It wasn't subject to the insane bubble that other neighborhoods have been (a half-million for a 1BR in bushwick? really?) and there hasn't been rampant overbuilding here - so there's not a lot of overpriced, empty units waiting to sell.
I think JH has some natural advantages that will keep it OK even in a slight to moderate downturn, the biggest being that it's the cheapest, nicest, closest real neighborhood to Manhattan. It's got a steady stream of people who want to live in JH, coming not only from Brooklyn and Manhattan, but from all over the world. It's still affordable to buy a decent place for couples who aren't in high-income finance jobs, but on teacher/fireman/bartender/police/receptionist salaries. Brooklyn's got some neighborhoods like that too (Kensington comes to mind) but that's a lot further out from the Midtown core of the city. JH is also really beautiful and has some great housing stock, both in and out of the Historic District. It's got great restaurants and street life - it really does feel like a 24-hour neighborhood (unlike, say, Inwood, which, while similarly priced, can feel desolate).
Of course, I'm just saying this because I'm in contract to buy a place in JH too (Just got the mortgage letter yesterday). But these thoughts keep me sane when I think about a housing downturn. Might prices be lower next year? Maybe. But mortgage rates might be higher, in anticipation of higher inflation rates in the future. And if you're not planning to buy and flip in two years, but are buying a home to live in, then you should be fine.
My two cents, at least.