I wouldn't say NASCAR is "underrated" so much as it is "over-hated"; mostly by people who think it's just "driving in a circle". The average person has no real idea how much work goes into building and tuning a stock car, nor the kind of physicality and focus it requires on the part of the driver.
That said, it can be a a bit of a chore to watch. My dad loves NASCAR, and even he used to record the races and play them back at double speed until there was a crash, or the final few laps.