I use GSF all the time, for brakes, filters, bearings, belts, heater matrix, exhaust... You name i've probably bought it from - I've never had any problems with their products apart from a faulty water pump - needless to say from the £100's i've spent there generally i'm very happy.
I only use the main dealer for parts that are unavailable, like quite recently; heater control cables..
I wouldn't worry about using GSF many of their parts come with a warrenty anyway.
Cheers