Regardless of level of play, many of you don't understand what an impact he will have on the Australian game. First of all, Sydney (and Melbourne especially...Adelaide and Perth both have quite a fair few Italians and other 'wogs' also) has a large Italian population and many of these people already follow Serie A & Azzurri or will be tempted to go to games just because of him. Second of all, he will be by far the best player to ever play in the league, there hasn't been anyone even close to his level before in the A-League so his ability alone will draw people in. With the previous part, there also hasn't really been a massive marquee signing like this before, Harry Kewell signed for Melbourne Victory last season but he is not even close to the international icon that Del Piero is. Dwight Yorke was probably the most successful marquee signing and Dwight Yorke vs Del Piero...come on.
Kids will be inspired by Del Piero and imagine what the players that play with and against him that have never played with/against a player of his class will learn. He really has the opportunity to leave a massive mark on the game here and as some have already said he could end up being the MLS Beckham of A-League. It really should be an invaluable signing for Australian football.
Also, you have to know that one of the main reasons of why the A-League isn't that popular is because the quality is lacking. There are huge amounts of EPL fans plus a good amount of La Liga, Serie A etc fans that don't take interest in the A-League because of the class difference. He can only increase the level of the A-League.