Longevity of plant propogation pad (ie, heating pad)?

Bought a "heating pad" plant propogation mat two seasons ago, it has worked like a champ and really makes a huge difference for starting seeds for me. I can get seeds germinating in three days, depending on the variety.
Here;s the question: Is there a governor on these things so they don't run for days and days? I check mine every day for sprouted veg, and the last couple days I've checked the pad, it has been stone cold. I could swear that last year it stayed warm overnight, as flats were warm in the AM.
I wonder what the lifetime of these things might be.. I started thinking about them when they were 14.95, bought mine at 19.95, now they are up to 27.95, and I wonder - are these things real pieces of junk?
Do the $70.00 versions last longer? I see people asking about medical heat pads 'cause they are cheaper, and I know that's a bad combo - water (seed flats) and electrical just don't play well together. I'm willing to fork it over (I think) if the thing will last more than 3 seasons..
Any suggestions? The pad I have was not purchased from these folks, but it looks like this: http://homeharvest.com/propagationaids.htm
Search for: Single-Tray, 17 watt version measures 9 inches by 19.5 inches and holds one standard propagation tray
That's the kind of mat I have questions about. Are the heavy duty mats better/last longer? Any safe alternatives?
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