Good but not great.
Positives first, the singers were entertaining and had good voices and great harmony. I absolutely loved their acapella rendition of that Smokey Robinson song. It was incredible...their harmonies were out of this world. They also were excellent during a gospel song they sang which I can't recall the name too. Both of these brought chills up and down my spine. The band was amazing as well.
Now the less positives. The singers do not have strong enough voices to be singing with such a great sounding band and most of their songs (except the two I mentioned above) were with the band. The one guy Phil has a great mid-range voice but from the very start he strains his voice to hit high notes out of his range.
Bottom line, they sound excellent acapella, the harmonies are superb, and the band is awesome, but in my opinion when you put them both together...it's just OK. I would recommend Human Nature goes either all acapella or just with some light accompaniment.