Nearly half the players drafted will probably not become good NFL players. Half of them will be busts, and only a couple will become elite players.
A “steal” can happen for a lot of different reasons, and time will tell what players in this draft are really steals; a lot goes into turning a prospect into a quality NFL player outside of when he is drafted. Still, a lot of uncontrollable things like problems in a player's personal life can have as much of an impact as controllable things like coaching and scheme.
All that said, there are always a few highly regarded prospects that fall in the draft unexpectedly and present a great value for the team that picks them. The 2013 NFL draft was full of twists and turns, and they presented several huge values for teams that let the draft come to them.