I saw one recent report that said you couldn't use it in the bars, but we've always been able to use it anywhere on the ship for our 3 previous cruises.
I wouldn't bother pre-ordering it. I think you can, but with NCL's new website, I don't know if that function is even working. Just go to any bar the first day and tell them you want to purchase the soda card and they'll charge it to your sign and sail account.
Last time we went, there was an additional $2 charge for the refillable "mug", which is a cup that looks about like a coke can, but I think holds less than 12 ounces. They also put a sticker on your sign and sail card -- and you just show that and then get your drink. Also, they normally won't give you canned soda as part of the program. It's usually limited to the soda dispensed at the bars or brought to you by your wait staff.