New double-pane windows make your house feel solid and quiet. You will notice the difference in your heating bill almost immediately once the drafty air stops sneaking through the old frames.
Aim for a fall installation. Contractors are often less busy once the summer rush fades, which can give you more leverage on price. It usually takes a few days for a full house job, so dry weather is essential to keep your home protected while the team works.
The 18-month intro window is a deadline, not free money. If you have a balance left over after that period, you will start paying interest on the remaining amount, so make sure you hit your monthly target.