Yes, first time purchases.
But you can use it more than once. Get your card during the 15% discount (normally 10%), buy a watch with the card. Pay off the card and then cancel the card. Wait six months and reapply. They will give you another first time purchase discount. I've done this after calling customer service, who told me about the waiting period. Granted, it can take some waiting if you already have a card.
It's possible it excludes Omega watches, but I looked for that exclusion in the fine print and did not see it this time. For anyone who is eligible and considering it, calling and asking customer service is a good idea. If it doesn't exclude Omega, you just need to get the card during the promotion, you don't need to make a purchase during the promotion period ( at least, you didn't when I used it.) You can wait to use the card for whenever the timing is best for you, as long as it's the first time.
They also have Longines and Oris.