In reply to Alison Parker:
I find the really cheap fleeces are more felted than the better fabrics. This tends to make them slightly more windproof, but less breathable. They bobble like crazy, too.
But the gap between bog-standard Polartec and 'cheaper' brands has narrowed, which is why Polartec have developed premium fleece fabrics such as ThermalPro and WindPro to distinguish themselves in the market place.
ThermalPro uses long fluffy fibres, which tends to make for a good warmth:weight & pack size, since it compresses well. I find it needs a windproof layer, though, as the wind whistles through it otherwise; that's a feature of the open structure of the fabric that's required to maximise trapped air.
Other manufacturers, such as Pontetorto and Terksaen make good fabrics (you'd be hard pressed to tell the difference between Terksaen and Polartec Classic).