I have the Campmor and it started leaking after a half dozen uses. Didn't pick the J&G model as it seems short as heck.

I bought a vinyl one from ebay, made out of hte same stuff rain poncho's are made from, which, while it's hot as heck, isn't much of a problem because it's so spread out and open air is under you.

This is not the same model, but similar to the one I got. I'd ask if there's an attachment to the bike--mine has a clip to afix the front to my handlebars (it's on the back side of the long, front cape section). This clip helps keep it from blowing out of the way.
http://cgi.ebay.com/Unisex-Poncho-Bi...item3360d49fb9

I'd also consider the Carradice model, even thought it's more money. It looks like a better design.
http://www.carradice.co.uk/index.php...&product_id=66