Anyone any experience of this:-

http://www.punctureseal.co.uk/

Ive used on on cycles and trailer/caravan wheels, but i wouldnt recommend it on a car. It goes in via the valve and can mess the valve up making it very hard to get air into the tire if you need to. And if it sets around the needle thing in the vlave it will cause the tire to loose pressure. Also it makes a right mess when it comes round to replacing your tires once worn out.