Wow, when I saw this, I though somebody wanted to get to know ME better!!
The AFCS in the CH47 (the only tandem I have flown) is, for a helicopter, excellent. It enables any pilot to pole a Chinook much the same as any other helicopter. Nothing is really any different (with the exception of ETL on the aft rotor during rapid spot turns)
HOWEVER, with the AFCS out they are PIGS to fly. Especially on instruments!
With the aft rotor producing the majority of the power, and very little inherent stability, the aft rotor constantly tries to overtake the forward rotor ANY WAY IT CAN!
I haven't got time to talk about 'Positive Stick Gradient', but any orthodox helicopter pilot would find the idea of bringing the cyclic back into their stomach, as the speed increases, not to mention, pushing it fully forward as the speed decreases, absolutely bizarre. (This is only AFCS out!)
All in all though, when it's working, it's the dog's bo**ocks.
As a footnote, anybody interested in Chinooks, may care to look at the thread in the 'Military Pilots' section, regarding a Royal Air Force accident on the Mull of Kintyre, Scotland. The 2 (tragically deceased) pilots need all the support they can get.