IMO, they're pretty average. There are better retellings of Arthurian fantasy, but there are also worse. I read them as a teenager in the 90s because they were one of the few series on the store's shelves, but I doubt I'd read them again with so much other and more accessible media available these days.

The only really interesting novel by Lawhead I've read was Byzantium, a story of Irish monks on a pilgrimage drawn into an extended series of adventures.