Glancing
at that very tree, and considering only NFL head coaches with whom he directly worked (e.g. not counting Saban's tree as his), it's mixed.
Successes:
- Flores (Miami)
- Vrabel (Tennessee; only played, not coached, under Belichick)
- Daboll (Giants), so far
Fine, I guess:
- Crennel (Cleveland, Kansas City)
- O'Brien (Houston), from whom the Easterby business springs
Busts:
- Mangini (Jets, Cleveland)
- Schwartz (Detroit)
- Patricia (Detroit)
- Judge (Giants)
Arguable both ways:
- Saban (Miami)
- McDaniels (Denver, Las Vegas so far)
I think the worst thing you could say is that Belichick mentees have done well in the NFL at about the same rate as everyone else has done well in the NFL, which is not very high.