I've had pretty good experiences with Dell's warranty service. I had 3 year on site service, and was always pleased. Granted, sometimes it took a while to get the service call placed (due to me taking my time and Dell's troubleshooting scripts the techs have to use), but every problem has always been resolved.
This may not speak very well for Dell quality, but in the 3 (4?) years I've had this laptop, I've had the keyboard replaced (may have been just a loose connection, but I paid for this warranty, damn it ), a new LCD (backlight would go off when it shouldn't), new hard drive and a new external power supply.
I usually use the e-mail support. You will generally go back and forth a few times (sometimes I don't think the people reading the e-mails like to read the *whole* message).
Dell's desktop warranty service is not bad either. As long as you can easily show what the problem is (i.e. bad hard drive using their diagnostic tools), that it.