Um, for those unfamiliar with UTH, it stands for Under Tank Heater and is the commonly used term for heatmat over in the USA. This cos they mostly use glass aquarium tanks for keeping snakes in.
The reason (I think) some people use a plastic floor mat in conjunction with a glass tank setup is that the mat itself can get pretty hot and if in direct contact with the glass bottom of a tank, it has been known for these to crack the glass, due to uneven heating. With a wooden or chipboard vivarium, you place the mat on the floor inside the viv and tape it down round the edge.
Under normal usage and provided it is not folded or bent too much, then there is no reason why a heatmat should not continue to function perfectly for years and years.
The one proviso I would add to that is that you should cover the area where the cable enters the mat with a piece of duct tape. This is the only part that is not completely waterproof. I know this, because, many years ago, Shaggy did a watery poop in that area and a few minutes later there was a bang and a puff of smoke where the liquid had got into it and shorted the cable and blew the fuse in the plug. So I had to get the snake out of the viv pronto, cos of the stinking smoke and then get a new heat mat... And that one I covered the connector area with tape (as well as taping it down round the edge anyway) and it is still being used today...
Edit: Oh and I forgot to say... The BEST way for a corn snake to keep warm is with a heatmat, because corn snakes do not bask under sunlight - they are a nocturnal snake - they warm themselves by coming out of hiding at dusk and sitting on warm ground previously heated by the sun.
Thus they get their heat through their bellies from the ground, not through their backs from the sun - so, obviously, a heatmat is a better way to warm them in captivity than a light, which imitates the sun.
A light in a corn snakes viv is for one reason and one only - and that is for YOU to see the snake better. The snake could not care less whether there is a light in there or not. And, unlike some other reptiles which need a UV light for health reasons, corn snakes do not need one.
_________________The Corn Snakes Sitehttp://www.snakepictures.co.uk/