Nakata is a superb player.
I saw him in the J-League when he was 17 and he scored some neat goals with his good control and technique. In fact he scored a similiar goal to Robbie Fowler's goal at Brann Bergen in the Cup-Winners Cup a few years back (flicked a long ball past a defender on the left flank, cut in and volleying past the keeper), and that was before Fowler scored it!
He was very good at Perugia but not at Roma simply because Roma players are unwilling to allow a player from a "3rd World Footballing Nation" to be so "big". Watch their games and you'll see the players reluctance to pass the ball to Nakata, unlike at Perugia where he was their starman.
I think Roma lost their head in the title race due to Nakata's signing, not due to Nakata's imcompetence, but to the selfish and big-headed players who refused to co-operate. They may say all the nice things during interviews but it became clear when the games were played.