I believe the Druids went even further than merely sacrificing criminals, as they were strict authoritarians (if Caesar's Gallic Wars is to be believed) and would also happily sacrifice dissenters.
But then, defying a Druid's authority would have been considered a criminal act anyway, thereby making all dissenters criminals, I suppose.
The Druids weren't above sacrificing the innocent, either, as (again, according to Caesar) human victims would be offered up to the gods in sacrifice as a substitution for the lives of those who might be going into battle and likely to lose their own lives (if not for having first appeased the gods with someone else's life). If all the criminals had previously been used up, then slaves would make handy sacrifices in such a case.
I hadn't heard of the Druid cannibalism thing before, either, and I wish the author of that piece had given a more specific reference to Pliny. All I could find in Pliny's Natural History was Pliny's mention of the Scythians as being a people who ate human flesh (Book 6, Chapter 20). I could have missed the Druid/cannibalism bit, though.
-- Nephele