I thought Centurion was relatively good, considering it seemed a somewhat low-budget film. The only thing is that because of it being an incredibly short movie, and not being a gigantic epic, it didn't really allow for characters to grow and other things to develop. But overall, I thought it was a good movie. Best movie ever? No. Good movie to throw on the TV while I hang around? Definitely!
As far as the historical basis goes, I didn't think it went TOTALLY crazy away from the real historical documents. Of course, no one really knows what happened to the legion after they went north of the wall (safe to say of course, they probably were utterly routed), but there are documents showing the legion being active in other regions a few years after (that's where it gets screwy), and THEN disappearing forever. So considering the real-life mystery of the situation, I thought it was properly handled, without a completely over-the-top story!
Anyways though, great action, good actors (a shame they didn't get to display their talent more!), solid plot.
But I must say- The Eagle looks like complete rubbish compared to Centurion. And Channing Tatum... meh...