Hmm, well of all the computer companies I've bought computer's from: Dell, Lenovo, Apple and Acer.
I can only give you my experience of two regarding Windows 8.
You've got to be careful when buying a computer with Windows 8 on it these days because it seems to me that since it's a new OS, alot of the bloatware that these companies put on the computers really slow things down. Examples include: I got my mom an Acer Touchscreen laptop with Windows 8 on it. The laptop wouldn't load webpages correctly and no matter how much Microsoft's customer support tried to help me fix the problem, the only way to fix it was to do a CLEAN install of Windows 8 on the computer with none of the manufacturers bloatware on it. Now it works like a dream.
Now my wife just got a Lenovo laptop with Windows 8 and it's having worse problems because of the heap of bloatware on it, running in the background and what not. Her system will hang when booting up or take FOREVER to startup which Windows 8 is not known for. It's a very fast OS. She's reluctant to do a clean system wipe but it'll have to be done sometime or another.
For these reasons, I suggest going with a custom PC manufacturer because they'll just install the clean OS on there, they aren't gonna clog it up with their own gimmicky software.
Unfortunately, I have no experience with buying from a custom pc manufacturer but they all have decent warranty programs, I'd say ibuypower.com has the better customer service from the looks of how they run their business. And I believe all the custom pc manufacturers will boot the computer to test before sending. You may have to ask the companies themselves of that policy though.
Either way, good luck!