Using a lunch bag (thermal bag) as a camera bag for my nikon d3000?

I was going to spend £40 on a bag for my camera but bought this product:-

http://www.polargear.co.uk/index.php/2011/06/polar-gear-lunch/

it takes my camera and a 55-200mm lense attached and was £3.

i assume the bag is meant to keep food or liquid at a steady temperature, ie. Will keep cool drinks cool and warm drinks warm, but if not used for anything other than my camera (never had any liquid or food in it) would or could it cause my camera damage.
I'm 99% sure its fine but would like confirmation. Why spend £40 when i can get one for £3,
i would only use the bag when out and leave the zip slightly open to stop any condensation building up

I await your answer.thanks

Added (1). Thanks Briar, the bag will be in a small rucksack, so should be fine, thanks for your quick respone, available in homebargains if anyone is intrested. But i do agree that the Proper camera bags are better, as in they have the removable padding to suit lense/body etc.

Shoiuld be OK provided it has the stiffness and padding to protect against knocks. Custom photo bags have very useful moveable padded partitions to separate the lenses, camera etc to stop them getting damaged by knocking/rubbing together when you move around.

That would be fine you do know I just dump my camera into whatever bag I normally use everyday. Also you don't need to worry about leaving is slightly open, all zips - even the best - let a little air in. The truth is, in winter you may actually have an added advantage with this bag as you said it keeps things warm, and a warm battery is a big advantage.

Camera bags are a gimmick (worst of all the branded ones), they just shout out "steal me." If you want serious protection get a Peli, if you don't need a bulletproof case, then any ordinary bag does the job.