+1 for calibers.
Ran one set for 6 full rounds at Aghalee (BADMCC) last season and they were stonking (they get better the more they wear!)
As Colin rightly says, they are slightly better on moist ground, and slightly skiddish when it gets really dry and dusty
... or as colin says, whens the track is Bournville they are good, when its Cadbury's its ok, and Lindt is just plain wrong
