We like others have said before wouldn't be without a barbag for touring. We use them for all our valuable items so that we can quickly take them off our bikes to take with us when shopping, visiting cafes etc.
We have the Ortlieb Ultimate bags which are great as they are waterproof and came with a great waterproof map case that is held on by poppers. I have also carried an SLR camera using the special padded insert that is available for them and had no problems with the camera.
Perhaps because we have always toured with them, unlike Tony's experience, we don't seem to find they have any real detrimental effect on the steering. If you have front panniers, they seem to balance each other out so to speak.