Anyone use oil pan heater?

[barnbridge]

I've seen several sites that sell a 5"x7" 500 watt stick-on electric oil pan heater for about $80. Anyone have any experience with them? Good idea or waste of money?
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[Arkapigdiesel]

I haven't used one, but I know someone that has one and he swears by the thing on cold mornings. These folks sell 'em http://www.wolverineheater.com/index.shtml

[Sageair]

I've used them on aircraft engines and they work great. One caution if you leave them on for days at a time without running the engine they can create considerable condensation inside the engine.

[jmh]

I have used one on my 99 Powerstroke 7.3 but only 150 watt. That along with the block heater on for 3-4 hours the oil would not feel cold to the touch. I will install one on my 05 soon.