Like suegill I've sometimes found I get good results just as easily with a camera - though it does depend on the camera, and you need to have a good light source.
However, my wife and I have fairly recently had experience with a hand-held rolling scanner as well. When researching these we noticed that most of them are unable to scan closer than about 1cm or so from the edge of the unit, and with some kinds of originals, especially books, that could mean part of the text was cut off. Eventually we found one, the Avision MiWand, which is designed to cut off only 2mm, and for our purposes (a large handwritten notebook with writing fairly close to the spine, and which wouldn't fit in a flatbed scanner) this was much better.
I don't know how much this applies to other scanners, but we found that it was sometimes quite hard to keep it rolling down the page in a dead straight line. If one end of the scanner moves faster than the other, the image is likely to be distorted, with some bits squashed together and others stretched out. This was particularly a problem if it was necessary to have one end of the scanner right up against the spine, as rubbing against the spine tended to slow that end down a bit. Having said that, with practice and some repeat scans, we found the end results perfectly acceptable for this kind of document, but it might not have been if there had been pictures, diagrams etc.
We also found it helpful, with this handwritten book where the text often went close to the top or bottom of the page as well as one or both sides, to have a sheet of stiff plastic underneath the page being scanned, so that the scanner could roll smoothly over the whole of the page, rather than dropping off the top or bottom edges.
Of course, being able to use this in a record office will depend on the staff agreeing to it - and with many kinds of document I can't help feeling that they wouldn't because it involves the scanner running over the document itself. In these cases a camera may be the only way to make your own copy.
Finally, while researching we found that there are a couple of flatbed scanners on the market specially designed for books, with the scanning glass extended almost to the edge of the unit, so that you can lay the spine along that edge and get the page flat on the scanner without damaging the spine. However, they are rather expensive.
Arthur
