Great thing to have
I have one, from West Marine, and I love it (although it was too expensive). As you say, it zips onto the forestay. It's also suspended off the deck, by the jib halyard - so the sail stays drier - and has a mesh bottom so that the sail can drain and breathe. If I change headsails, like Herb Parsons I use the 'deck' bag for storage instead of the regular sailbag (I only change sails before I go out).I also have a downhaul connected to the head of my headsail, down through the hanks, over a block at the tack, and leading back to the cockpit. Between the bag and the downhaul, it's much more practical for me to sail single-handed. And it reduces the time I have to spend pre- and post- sail. Deck Bag + Downhaul = Poor Man's Roller Furling!The only drawback: at the end of the season, I notice that my headsail is rumpled and crumpled: not smooth and wrinkle-free as it would have been if I'd flaked it after each sail.Dick