I quite like Valygrl's idea about using it as a pillow. I put it in a stuff sack to see how small it would go, and I could it down to about three inches by six inches by six inches, and only 650g. I know that's heavier than an inflatable pillow, but I've used those before, and needn't have bothered, it was so uncomfortable - well, not as uncomfortable as a pannier. I think it's a great way of carrying a comfortable pillow, and having backup should you need something warm to wear or sleep in. In the UK, even in May you can get a cold snap where something like that would be invaluable.
I've decided I'm definitely going to keep it, especially seeing as it was so cheap. At first I wondered about using it just for its filling - I have a down bag that's nice, but could do with filling a bit more in my opinion to take it from a 2 season to three season. I'm a dab hand with a sewing machine, so figured it wouldn't be that difficult (though messy). And the jacket was actually cheaper and a better down to feather ratio than the cheapest down pillow I could get (which was £30 - you woudn't believe it but I simply can't buy raw down anywhere). But after wearing it a bit, it seems so much more flexible to keep as a jacket - can wear it in camp at night, can put it between sleeping bag and pertex cover if a bit chilly, or just wear it if the temp really drops. And like Valygirl advises, use as a pillow when not in use.