Having taught one glider pilots powered flying from his first trial lesson and flown with one other former glider pilot on the navigation stage of his ppl training then I would say gliding is an exellent foundation for powered flying. The glider pilot I taught was landing the aircraft the first session in the circuit, it was only UK weather that delayed his first solo. The other had nearly completed the whole course in 25 hours. It is a shame that the JAA system phased out the silver c conversions that allowed glider pilots to obtain a ppl with only 20 hours of training.