He sucks, bottom line.
Every quarterback in the NFL, even third stringers, are capable of getting into a rhythm and making some good passes. Sanchez does this rarely, and when he shows signs of competence- not elite skills- he is praised like Tom Brady x Peyton Manning.
Jets fans love to compare his development to great quarterbacks in their first few years, acting like he has already been given the right to greatness. Well, looking at his numbers, you'll find that he's much, much, much more likely to end up like Alex Smith, Tim Couch, Brady Quinn, or Ryan Leaf.
Last year I posted an in-depth analysis of the "Peyton Manning Myth", which basically showed that, while Manning underwhelmed in his first season and went on to greatness, he is the exception and not the rule. Most QBs are what they are. Sure, they will improve from their rookie season, but the there is rarely the type of enormous development that is fantasized about. QBs that throw a ton of INTs or have a really low completion % almost never succeed, despite all of the "potential" labels given to them. Manning is basically the only exception, and his situation was a really horrible team, nothing like the Jets.
The quarterback who had a comparable role to Sanchez was Ben Roethlisberger. Big Ben had a great running game and defense his first year with the Steelers, some good receivers, and was basically a game manager who was also supposed to make plays when needed.
The difference? Roethlisberger delivered, putting up a QB rating of over 98, with 17 TDs and 11 INTs, and leading the Steelers to a 15-1 record his first season and a SB victory in his second. Also blessed with one of the league's best offensive lines, defenses, and running games, Sanchez was one of the league's worst quarterbacks last year, putting up 12 TDs and 20 INTs.
He'll do better than Monday night, and there will be flashes where he seems like an average, or even good, quarterback. But I guarantee this guy is a backup within two years.