I bought mine from Costco on the release day.
There are definitely some advantages to getting a smartphone at Costco. The 90-day exchange is great, and they will honor it, no questions asked. People have brought back phones with smashed screens within 90 days and gotten replacements. And you still have Verizon's 1-year warranty...the Costco 90-day period doesn't make VZW's warranty null and void. If you have a Costco Amex, you're getting cash back on that $290 purchase. And while the accessory pack for the Nexus is kinda lame, they have given out better accessory packs for other phones, and I won't turn down extra stuff, even if it's crappy. The horizontal pouch case is actually exactly what I was looking for since I carry my phone on a belt while I'm at work. But the biggest advantage for me as a CA resident was that Costco taxes you on $289, whereas VZW charges tax on the full retail value of the phone ($750 or something). I believe this is specific to California.
Costco will not honor a NE2 discount or work discounts that VZW will. I lost my NE2 by purchasing at Costco, and while I can still use my $50 work discount for smartphones if I buy my next one directly from VZW 2 years in the future, I couldn't apply it to the Costco price. So actually the phone would have been cheaper for me to buy at VZW if it weren't for the tax thing. With the cheaper tax at Costco but the more expensive price without my discount, it actually worked out to be almost exactly the same price from both places, but without the accessories if I bought directly from VZW.
They do not have a 30-day return policy like someone mentioned earlier. It's 14 days, and if you bring the phone back even one day later to return it rather than to exchange it, you can't get your contract reversed and will only be able to buy phones at full retail price for the rest of your contract. I found this out the hard way 12/30, 15 days into my contract, when I tried to return it because of a major problem. Also, if you return the phone, you also have to return everything that came with it, including all chargers/headphones and the accessory pack. I'm not sure how this works if you exchange rather than return it.
Costco also gets only limited quantities of each phone. My local store had 6 Galaxy Nexus phones. I bought the last one on 12/15, and it hasn't been restocked since then. This has created a big problem for me, since I got a defective one and need to exchange it, but I can't find a Costco that still has it in stock. I even looked at two Costcos in a state 3000 miles away since I was there on vacation.
I hope this helps clear up some of the issues about buying from Costco. I think they're a pretty good place to buy a phone, despite the limited selection.