Hi Simon,
When I switched to Mac I bought the jampacks to use with garageband before I decided to shell out for Logic Pro. To be honest, I think they are O.K, but there is a slight issue with some of the levels in the orchestral instruments (way to high) but that is not a biggy. As Martin said, the loops are everywhere. East West put out some massive libraries of Apple Loops and REX files which IMHO are of a higher quality than the Jampacks with huge variety.