I personally try to avoid it. I have only bought from sellers twice who used the GSP, and both times the shipping service poorly re-packaged the ponies. They were more expensive ponies too so it was disappointing that there was not more care in packing them, considering it was nearly triple the cost to mail them with the GSP. One box arrived crushed and open. I called eBay and they said that as long as the item arrived undamaged, they cannot do anything about the packing. I was a bit gutted they did not seem to address the poor packing of the GSP. So from a buyer's standpoint, I have chosen not to bid on items that use the GSP.
I have not received any customs fees either, but I am unsure if the GSP has anything to do with that, or if it varies by country/cost and type of item. (I am in the USA). I believe as a seller, the GSP is supposed to offer more security. I'd be curious to hear a seller's perspective on this though.