We use luan or paneling for ceiling in basements where drop ceiling tiles are too expensive to be practical. I hate drywall ceilings in basements because cutting into them for access is an eventuality. With the paneling/luan screwed in place, it is only slightly more work for access than drop ceiling tiles, just have to remove a few screws.
The problem, as noted, is sagging. We use furring strips on top of the sheets, perpendicular to the joists(rafters in your case) to support the material, usually 24" on center. It tends to have a rustic look to it, we have done both painted luan/smooth wood styles and paneling/stained rough cut wood styles. You can do just long perpendicular runners, or make a grid pattern for aesthetics, I've found both to look decent. We've never had panel sag once these strips are in place. It's important if you do strips in just one direction to go perpendicular though - the screws into the joists will support the material in one direction, the strips are there to prevent sagging between the ceiling cavities in the other direction. Store bought el-cheapo furring strips are a little too thin for my tastes (and I don't like the rolled edges), so we usually rip ours about 3/4" - 1" thick from dimensional lumber or rough saw planks.