"The fact sheet claims agencies spend over $1 billion a year on PCs, and says nearly $6 billion worth of purchases have funneled through the GSA over the past decade - delivering an average savings of 38 percent compared to other procurement channels."
We have a single federal government agency for non-trivial procurements in Canada. The had a standing offer arrangement for computers and software. From what I saw, it seems doubtful that the savings, if any, were any where as near as large as what was claimed. One thing I noticed was that the stuff on the price lists tended to be dated, and when prices were falling (which they were up till recent times), It seemed you were locked into both old prices and old technology. Moreover, if you were looking for anything that wasn't completely standard, you were likely to get gouged.