I investigated buying an iPhone, so I can only answer on possible issues.
Often the US is not as cheap as it first appears. First, in the US prices are published without taxes. Secondly, if the total value of the goods you bring back are more than EUR 450 you'll need to pay custom taxes. You also need to buy a local charger or plug adapter.
In the case of an iPhone, the price was still lower in the US, but it was more in the 20-40 eur range. This plus the limited support if the phone turned out to be defective made me decide not to buy an iPhone in the US.
I also ran into issues by ordering online, with the objective to let it be delivered, while in the US. This turned out to be impossible in many cases. Some online shops only accept American credit cards and most that do accept foreign cards, only will deliver to your home address.
Personally, I think that buying electronics in the US is only beneficial if you can accept the limited warranty and if you are able to deduct the customs taxes from your local taxes (e.g. tax deductions in case of running your own company).
While I was in the US, someone I met went to Best Buy, to investigate the Nexus. I don't know if he ended up buying one. I went to Best Buy myself and was able to pay with a foreign card. I recall that we discussed bringing our newly bought phones back as if it was our phone before going to the US, i.e. throwing away all receipts and packaging. Again throwing away receipts might limit your warranty even more, but more importantly, customs apparently do check serial numbers on some occasions.
As said, I ended up not buying an iPhone, but I did buy up to 200 dollars of electronics, saving me about 45 eur versus if I had bought it at home.