My original Apple MagSafe power adapter died after 3 years of hard work traveling all over the World, feeding off the outlets I wouldn't want to describe spikes and electric current quality. It finally died last spring.
I bought an aftermarket adapter off eBay for around 24 bucks and immediately noticed it is different than original Apple. But it worked (for a while at least).
In November 2009 I have noticed that my aftermarket MagSafe started to get tooooo darn hot shortly after I plug it in. I promptly ordered another aftermarket adapter from eBay. Now it is 17 bucks delivered.
Started using new adapter in December and in January noticed very high adapter temperature again.
Yesterday it nearly blew itself apart. Something blew inside really loud.
It was still working for another 20 minutes with very strange buzz.

I took it apart (as I have an old spare half-broken adapter to use). Here's some pictures of the latest blown adapter:



it fall apart really easy, I had to pry it with screwdriver just a little as plastic was cracked here and there because of the temperature.
Looking at the back of the board I found small resistor (or something) near low voltage feed blown (it is visible sorta reddish on the image above).



As you can see the whole board around transistor with radiator plate became reddish as a direct result of exposure to constant high temperature. I do not have a thermometer but I can tell - box was hotter than 200F. I no longer felt safe to leave it plugged overnight.

Let's do the math:
Apple original MagSafe: $80 - good for three years
Aftermarket MagUnSafe: $17 every 6 months ($34/year or $102 in three years)
I feel like buying cheaper Apple original MagSafe for $80, at least it died quietly without making too much noise.There's no place like ~

I was looking for an aftermarket adapter some times ago.
Bought one on eBay - lasted 3 weeks before blowing-up. Good thing I was home.