It really depends on what you are looking for. We stayed in Williamsburg for the duration of our trip (about 5 days) to get a feel for it, as lots of people recommended it. It turns out we didn't really like it (nothing bad per se, just not what we're looking for).
We settled on Lower East Side, as it best represents what we're looking for.
To handle the issue of apartment hunting while not living there, we're going to end up just using airbnb to find a monthly rental, move our stuff up and put it in storage (we're moving from DC), and hopefully will be able to find a place easier by being there.
We're going to live in Brooklyn for sure. My girlfriend likes Brighton Beach, and I'm from Russia, originally, so I wouldn't mind living there. We also heard great things about that hipster part of town whose name I forget.
We should probably combine forces somehow.
I hope they eventually expand too, but from what I gather the problems in NYC are vastly magnified because of a) how tight the market is due to height control restrictions on housing and b) the sheer cost of housing. I'm living in Tucson, AZ, right now and paying about $550 for a pretty nice one bedroom that would probably cost $2K in Manhattan and 1,400 in Brooklyn. If this one doesn't work out, I break the lease for $1,100, and although it sucks, I can find another place of similar quality relatively easily and am out a lot of money but not a catastrophic amount.
If I do the same in New York, I'm out a TON of money and have to do a lot of shopping, at least from what I understand.
Also, hoping they expand into other areas; no reason to limit this to NYC. I've got a couple of places in Dallas I'd gladly register to bitch about.