In all fairness I'm not sure it's so much 20 something tech guys, even well-paid ones, who are the primary audience for these. I suspect it's far more empty nesters who don't need or want to deal with a house any longer. Or execs etc. for whom this is their in-city residence.

That said, I'm not in real estate and don't have a good feel for how speculative all this is. The Seaport area in particular is apparently very hot right now in terms of real estate prices even though it's still something of a work in progress.