Worked for a contractor who does tech support for a company that buys all IBM. I used to call IBM quite a bit on warranty stuff. 95% of the time I knew what the problem with the pc was when I called, dead hard drive, bad ram etc. It's when the problem is with the motherboard you can't be absolutely positive a lot of the time unless you've got the exact same model sitting by thta you can swap parts with. When that was the case I usually walked through all the trouble shooting steps I took with their tech and if they had something else to suggest I try I would do it. How hard is it to try something while they're on the phone with you? Not very!
Towards the end of my job as a contractor there I did find the knowledgability of the techs I was getting on the phone went down. The last couple times I called I could tell it was someone with no tech experience reading from a list of things to have me try. If anything different than what the list said to expect occurred I would get put on hold and then someone from their "technical support" department would get on with me. First you had to get past the meat head though, lol. The technical support guys can usually tell when you know what you're talking about and would put the case number through for us to rma on. The also knew that as an authorized IBM service provider we weren't going to claim something was bad when it wasn't because the defective parts would be shipped back once the new part arrived, and if the claimed defective part was actually good, we would be charged a penalty fee.