As 05mystang said, They are probably the best bang for your buck mustang in existence. Tons of raw power, fairly reliable, great handling. And most importantly, 20 years from now the value of them will be going up and up. It'll be like finding an oldie with the 427 Cobra Jet in it. People always latch onto those special powerful mustangs, and the 03-04 Cobra "Terminators" will do nothing but value themselves. They are by far the best mustang in terms of bang for your buck.
That being said, the reason you are seeing so many, is that they aren't your standard daily driving cars. People are having to sell of their toys to make ends meet. Since Cobra's aren't practical (For reasons of fuel mileage) as DD's, they have to be sold. If you can afford to attain it, go for it!
2004 Sonic Blue V6 Automatic ~ Daily Driver
(Standard Bolt-On's) 20 MPG
2003 Torch Red GT Convertible Manual ~ Toy
(Looks good, Sounds Good) 20k miles
You make Lemonade, I'll throw the lemons back at your face.