The Porsche GT1 by scaly (club or other editions) are a handfull w/o a magnet esp with stock tyres. However they are good to learn basic no mag driving skills in as they are a challenge to drive fast (if you keep them in stock form) and you really notice the difference in lap times if you drive smooth, keep it in a straight line and only accelerate when you are pointing in a straight line.
The most forgiving way to learn no mag is with a SCX rally car and these can be picked up for not huge bucks.
However speed is much lower with no mag so even if you do come off you'll be suprised at the lack of damage inflicted ie any car is a good one to learn with.
Matt