My own view is that the Belgariad would be terrible campaign.
It would be subject to all the criticisim that is normally levelled at FR: the NPCs are simply too powerful and overshadow the PCs. And if they don't, then you're looking at tremendously overpowered PCs. Neither situation provides the basis for a long-term campaign.
Also, the story of that world has pretty much been covered by the ten books.
If you really want to adapt a world from a novel, why not try Raymond Feist's Midkemia? After all, that started as a game world and seems to have a lot of backstory that would explain a number of interesting sites etc.... Also, while the NPCs are powerful, they're not "off the scale" compared to high level PCs.
Cheers
NPP