I have the Zbook 15 G3, When I first was able to spec the machine of course I chose the quad core i7 with 16Gb ram a 1tb hard drive and all the bells and whistles.... Who wouldn't. I was thinking .... yea finally a Ferrari... but to my surprise, I actually got a YUGO with a chevy 350 welded into the engine bay, with only about 3 or 4 of the cylinders working.
Its been a problem child since day one. My Zbook has been in the hands of IT or HP for more weeks than I was ever able to use it. Its had the video card replaced, and about 3 or 4 hard drives by now as well as the power supply replaced. Just got it back last week and it failed again. It choked when I opened my 4th minor excel spreadsheet and shut down Excel. Sometimes it will run as slow as a 286 and then other times it will breeze along as one would expect a quad core i7 to do.
On the positive side, it has the heft of a 1942 Buick Roadmaster, so it makes for an excellent defensive weapon.
I have a couple of elite books which are fabulous and reliable machines (840 and 850). The 850 is the standby machine I use when my Zbook is in the shop (which is most of the time).
If One is considering a Zbook ... One should consider the elitebook instead and get an external CD/DVD if necessary..... I cannot recommend ever acquiring a Zbook.