Hi East Ender. Pretty much agree with all of Liquid Gold's points but I'll throw my 2 cents in as well. Willis works hard and will rarely put in a half-hearted performance. As mentioned, athletically he's top draw. He's quick, he can jump and he's quite strong for his size. Injuries used to be a big problem for him during the early part of his career but that's seemed to have corrected itself somewhat - although he's still susceptible to picking up the odd niggle. In my opinion, and I've said this numerous times on this forum before, if anything lets him down it will be his height. Standing at 5"11 he's certainly not the biggest CB and as a result he's struggled at times with the physical side of the game - especially from set pieces. Whilst his athleticism has made up for that in League One and Two I can only speculate whether that will be enough to make sure it's not an issue in the Championship. If playing against physical strikers such as Vokes, McBurnie or Jutkiewicz then he'll probably be caught out. Also, he's okay on the ball but not the best, hence why he's poor at RB. (If he's positioned there, know that he'll refuse to go forward).
He'll be a cheap option for you though so it'll be fairly low risk I suspect.