We had a stackpack style cover made and after a few modifications I really like it. There are a few negatives which I'll list first. The cover remains in place while sailing and on the windward side it bellows out a bit, never flaps or makes noise but obviously disrupts air flow somewhat. At the mast the cover attaches to the mast so you have to undo it to access the reefing hooks, not a big deal but makes it fractionally more difficult to hook the sail. On the plus side it's super easy to use, just zip it open to raise the main and when you drop it just pull in the reefing lines and zip it back up. Couldn't be simpler which means that we use the main more often than we used to for short sails and we also cover the main whenever we aren't using it (we used to leave the cover off while cruising unless we were anchored up somewhere for a few days). I wouldn't bother with one if I was going offshore, but for coastal cruising I wouldn't go back to a normal main cover.
SV Laurie Anne
1988 Brewer 40 Pilothouse