I think with any of these rookies, you can't tell much until about 3 years in. Everybody thought Kaepernick was going to be something after his first year, but people got film on him and he had a coaching change and that was that. Elway and Manning looked like shit their first year, but really started blowing up their second year. Reserve judgement until year three.
My prediction is that Mayfield is a quality starter for years, one of the other 4 drafted in the first round is as well (don't know which one), one of the other 4 has a career shortened by injury, one of the other 4 is a career back-up, and the last of the 4 is a bust.