I just returned home from my first visit to the Manhattan Costco.
I'm used to shopping at the Yonkers store (with a friend who owns a car) and the new store seemed quite a bit smaller - sort of Costco Junior. It felt like there was not as much variety as in other Costco stores. But I'm really happy we have one here and it's still way better than my local D'Agostino or Gristedes.
Of course, everything is still the usual Costco mass quantities. I took the M15 bus and brought along my wheeled weekend bag to get everything home - the M15 downtown on Second Avenue is a few long blocks from the store, so wheels helped a lot. Worked out fine, although I had to be very careful not to overshop.
The neighborhood is okay, but if you're taking the subway, as neuron suggested, you might want to take the M116 bus to the store, since the 6 train stop is at Lex and Costco is east of Pleasant Avenue (which is east of First Avenue).