Dell turned us off with our institutional purchases. They kept on getting the custom configurations wrong, but curiously it was always in their favour. After a while we dropped Dell completely. This kinda surprised me actually because I had always assumed Dell targeted the corporate market. Maybe it was just a local thing I dunno, but it was horrible.
With Sony our only experience was with personal consumer oriented computers, but that was also horrible. Weirdly enough, I've had good experiences with Asus (Nexus 7), and Acer (el cheapo nettop).
With Apple, the support has definitely been good for me. I'm a bit biased of late though, because my currrent iMac is a replacement from Apple. I had a 2009 iMac with 4x2 GB = 8 GB RAM. That thing was having problems so I brought it in. They tried to fix it. Didn't work. They tried again, didn't work. So, they offered to replace it with a 2010 model. They transferred the data to the new iMac, AND removed the old drive from the old iMac and handed it to me to keep. Unfortunately, the new computer only had 2x2 GB because that was my original purchase. The other 2x2 was an aftermarket addition. Nonetheless I had informed them of that when I brought in the computer so they took back the computer again and got me ANOTHER new 2010 iMac. In fact, the called me to ask if I wanted 4x2 GB or 2x4 GB and of course I took the latter, not only because the latter left me two memory slots open, but also because at the time 2x4 GB was much more expensive. This whole process did take a few weeks, but I came out of it very satisfied. Brand new and faster machine with 2 more open memory slots than I had before, for no cost to me.
Actually, because it was a new machine, they couldn't transfer the Apple Care warranty, so they refunded me the remaining amount of the Apple Care. I could then choose to buy Apple Care or not again for the new machine.