well, I'll tell from a personal experience that is going on right now.
I got my Xbox during Christmas. It seemed to run fine, but then I kept getting a disk reading error. I didn't mind, because I rarely touched the thing.
Then Fable came out last week. I play for hours at a time and then I get that message, and it's been pissing me off. I decided to research what was wrong.
When the Xbox was first released, it was released with a Thompson model DVD drive. It had an insane defectivecy rate, and Microsoft never claimed responsibility. Instead, they silently switched it over to a Phillips in August of 2002 and over to a Samsung in September of the same year. The Thompson still comes in Christmas packages though, because it saves cash.
So I got my Xbox in a Christmas package with a defective Thompson drive. I found out that I have to open up the Xbox, open the drive, and replace the laser. The laser is coming in the mail, and I will have it fixed soon.