Remember Friendster from 2003? It's quite possible that it would have not lost to Tribe/Myspace/Facebook if it didn't take minutes to connect before eventually timing out.
There are things that are worse than wasted effort, and one of them is a hockey stick that slips right through your fingers. By the time the curve is inflecting, if you can't keep up, you're burning customers and inviting competition. If you have to rewrite at that point, you are going to lose to competitors with deeper pockets and no PR baggage.
Poorly-designed hit any day. Was this a trick question? :-)
(Obv, I'd prefer a well-designed hit, but I'd rather move fast and validate my market with a hack, than spend too much time and energy building something good that'll never be used.)
I'll take the market hit any day! (Anyone remember the fairly pervasive "fail whales" along Twitter's bumpy evolution? Did they negatively impact the end state of the company?)
Let's ask ourselves, as either stakeholders or users of Twitter or participants in the culture borne of Twitter's poorly-designed success: are we happy with Twitter's arc?
I would rather have a Basecamp level of success that was well-designed and sustainable. And you don't get there by putting out a pile of shit and hoping for a runaway hit and then scrambling to raise billions so you can backfill everything you neglected to think about in the first place. The only ones who want that are VCs and the hyper-ambitious entrepreneurs who would rather have a billion dollars than make the world a better place.