The problem with the onsite service is it is not done by Dell, it is done by a third party (it says so right on the order page)
It is kind of spotty, either you get lucky and get someone who has a clue or you don't.
I had one customer, they had a bad hard drive in their desktop computer. The tech came in, replaced the hard drive but didn't load windows on it, he said that you had to do that yourself. He also didn't save any of the information on the computer!!!
He tried to take the bad hard drive with him and they had to fight him to keep it.
Since the information for their business was on there they ended up paying us several hundred dollars to get their data back.
Pretty useless "on site service".