In that early game against Geelong last season PJ was moved onto Cam Mooney sometime in the second half. Mooney had been dominating and Johnson, while not beating him outright at least kept him quiet.
People say he isn't fast but on that day he kept pace with Mooney and was able to spoil and harass quite effectively.
I thought then that he could make an effective backman - still do. (Or to put it another way - could be used effectively on the backline)
Johnson is to my mind a more rounded player than Jamar offering the team more options, forward, back and in the ruck.
I noticed last season how well he presented himself at kick ins and how the kicker often took the option of going for Johnson.
Obviously a sign that his team mates had confidence in him and his marking.
In 2006 his overhead marking was (to say the least) poor. Last season it improved dramatically. He is still not a strong overhead mark, however given that improvement between the 06 and 07 seasons I believe there is every reason to be hopeful of further improvement leading up to the 2008 season.