Just to join the throng of bloated body beaters:

It still seems preposterous to me to assume that there were enough erratics to create the circle, leaving no other bluestones throughout the rest of the region.
Also, why, when we know that our damned clever ancestors heaved forty ton blocks for miles uphill (Old Keig), or along the coast (Rudston), do we find it so implausible that they made the extra effort for what was clearly one of the most important sites in Britain?