"why don't you build your own machines? Twat."
Well, if answering ad hominem and calling people twats to make your point is your level of competence, then I can see why you're posting anonymously. And I don't need to build systems myself (I could, but not at cost -- volume buying, see?) if I can order them at cost from a variety of companies other than Dell. And I can and do.
As for the details on the switches to XServe, the pre-press business had heat and maintenance issues with their Dells, possibly related. After several replacements, they decided to try something else. The engineering firm had wanted to switch to a unified server platform to save on maintenance, tried a rackful of brand-new Dell units and decided to hand them back after having found them to be wanting in several areas, including performance per watt and maintenance cost. They ran their PPC970 XServes on Debian GNU/Linux, last I heard.
I hope this satisfies your need for specifics.
"... over the course of my 15 years of professional experience only a handful of times have I had an issue they haven't fixed (and they were IBMs)."
Interestingly, I have always found IBM's service to be quick, friendly and thorough on the very rare occasions that I actually needed it, which is more than I can say for my experience with Dell. HP I have usually found satisfactory. Sun has set up a service bureaucracy that is almost impossible to penetrate at times; try getting a price quote on a spare part from them without signing an order form...
Just FYI, I am not a particular fan or antagonist of any platform or hardware supplier. I use what works. If something doesn't, I go get something that does.