A good mechanical keyboard fixes hand fatigue and lasts for years, meaning you buy it once instead of replacing cheap plastic boards every couple of years. Look for hot-swappable switches and a solid aluminum frame so the hardware keeps up with your daily typing.
Watch the release cycles before you buy. Brands often drop updated versions in the fall, which is when last year's exact model usually dips in price without losing any real performance.
That intro window is a strict deadline, not free money. If any balance is still sitting on the card when the 15 months run out, you will owe interest on the original amount from day one.