I agree with TEC, in that Pietersen can't be called an excellent player of spin based on one series when he faced just one spinner, albeit a superb spinner. Take a player like Lara - he's considered by many to be the greatest player of spin, but that's not due to one series. That's due to him consistently belting the likes of Murali, Warne and all other posers.
To use another West Indian example, Sarwan had a very good series in Sri Lanka a few years back, but I still consider him a competent/good player of spin, seeing as he hasn't managed to back that up consistently against quality spin.
You can't say a player has arrived if he has only played one series. Yes, Pietersen showed excellent capability in the Ashes and played a good series, but considering that there are more teams in the world than simply Australia, he's not even faced half the challenge yet.