How about one of these instead?

http://www.montbell.us/products/disp.php?cat_id=71&p_id=2301287

Heavier (about 2oz)
Less compressable a bit - I got one of these down to a sandwich size. Easily small enough to drop in a fanny pack with room left over for your EDC junk.
More expensive - but quality usually is.

But a real live insulated jacket that will last a long time and really actually keep you warm.

And as far as Montbell goes, they are a high-customer service, cutting edge company. I own a few of their items and love them.

Here is a slightly heavier option that I also frequently carry in my daypack:

http://www.moosejaw.com/moosejaw/product.asp?s_id=0&dept_id=10165&pf_id=10023724

Stuffs down to the size of a 12-inch softball.

I carry both in fully waterproof silnylon stuffsacks.