The worst situation I've ever had was with a rackful of Dell servers I purchased. Almost immediately after delivery, many of the machines in the rack needed their motherboards and risers replaced. Fortunately it was very easy to see on booting the machines straight away that these were sick. Obviously a major QC failure.
Usually I'd have had them replaced but I was out of time. Since I was on the same-day service on all the machines, Dell sent several engineers to make sure that all the machines were up and working on the same day. Unfortunately, I had a similar problem the day after with more machines and they couldn't turn up on the same day - so the problems were fixed the day after.
I was hopping mad and while I let the engineers get on with things I made my extreme displeasure known to my account manager, both of the failures and of the failure to meet the same day support requirement. Dell eventually sent me a free license of an add-on software for the backup subsystems as compensation. I guess it was better than a kick in the teeth.
However to their credit that was nearly 2 years ago and all the servers have been trucking on with only a couple of fried PSU's (which are redundant on each machine anyway) and a handful of failed hard disks (once again, RAID5) between them, despite being moved around various locations.
It's safe to say though that Apple has presented me with the worst service and hardware in a long time of buying personal computers. This year has certainly been the most disruptive in I'd say at least 6 years. I don't buy cheap - i.e. I'm not foolish enough to go for a 6 month warranty on a bare-bones machine and compare it against an Applecare'd Apple for example - but I buy fully covered higher-end machines for most manufacturers. On that basis of that I've had no major issues with non-Apple desktop / laptop hardware during that time.