I find onsite support to be a crap shoot. I've had Dell and Lenovo onsite service multiple times. Lenovo is better though in that I've convinced them more than once to trust my diagnosis and just send me a part. But the on-site techs, when they show up, are frequently not skilled and make mistakes. I've more than once had to instruct them how to correctly do their job. FYI this was service on thinkpads, thinkstations, and dell precision laptops.
My experience with dell and lenovo has been the exact opposite, they tend to know more than I do.
Maybe it's my proximity to silicon valley.