Clive, Ultraseal is pretty good now, as opposed to when these products first came on the market.
It will repair a normal type puncture, like a nail through the tyre, but will not prevent a full blown "Destructive Blowout" of a tyre.
The other way it helps is by making the tyre run a bit cooler than normal, which is beneficial to the rubber.
I use this product on my van, car and mountain bike, and have found it invaluable.