I've had the Capital One for years. I used to love it, but have my eyes out for a new one because they have lowered benefits/raised miles needed a number of times. Other cards give more cash back than Capital One's miles credit or cash back, so it would make better sense to use one of those and redeem for cash to pay for the flights or whatever else.
That said, it's a snap to redeem the Capital One miles, there are no restrictions, and they'll even credit you after the fact, even some months after the fact.