I really enjoyed these books. Have read them all but No. 6: The Sword of the Lady. (Waiting to get my Nook e-reader which is supposed to ship sometime in January...)
My only complaint about the series is that the Wicca thing got a little heavy handed after a while. I think I understand why he used it -- he needed a way to get the supernatural/magic into a contemporary American society, so why not validate real life witches? I personally just did not need that much info about the religious philosophy of Wiccans. Not that I have a problem with it as a religion, it just interrupted the story at times.
If you liked this series, you should also read Stirling's Island in the Sea of Time and the follow ups. They were written first but are companion novels to Dies the Fire, etc. They are the flip side really because they tell the story of what happened to Nantucket when the "Change" occurred: the island, all its inhabitants, and Coast Guard ship are hurled back in time to the Bronze Age. Fun stuff.
I expect Sword of the Lady to tie up the story between Dies the Fire and Island in the Sea of Time, so if someone was going to read these, I would say start with the Island trilogy first.