Most people haver never owned and used a true professional-level camera. The difference in performance, reliability, price, and weight between a true professional-level camera and an introductory-level or intermediate-level model camera is pretty significant. The 5D and 5D Mark II are basically fairly unusual cameras: based around a roughly $389 EOS Elan film body design, with almost ZERO advanced body features and zero advanced sub-systems, the 5D and 5D-II have offered Canon's "compact, light weight, low-cost full frame option". A cheap to produce, very simplified camera body, fitted with an excellent sensor. No remote flash command system. Color-blind light metering. A truly bare-bones control layout. Very minimal customization of controls and settings (does not have for example Banks A,B,C,and D with all banks independently pre-programmable,for example). Simplified autofocus system.
There are situations where a "pro-level body" makes getting the required kind of photographs fairly easy, and where a slowish, lower-end camera body can have some hesitations and focusing failures that make the camera not "bad", but simply un-reliable, or not-dependable, on a shot after shot after shot, hours-long session basis. People saying a better camera will not make you a better photographer...you know, that is actually not 100 percent true, through and through...that's more of a cliche than an actual real-world fact. When confronted with a CHALLENGING SET of circumstances, the world's better cameras are actually significantly better tools in terms of being able to GET THE SHOT WITH ALMOST CERTAINTY. That's the difference between shooting a pro Nikon or a pro-level Canon body, and a mid-level body, which is what the 5D-II is at most...:"a mid-level body, with a fantastic SENSOR,and a good but not great autofocus system."
For many situations that are not demanding ones, the superior sensor performance and light weight and affordable price of the 5D-II is a totally winning combination,and one used by many Canon pros who want Canon's "lightweight, affordable, low-cost full-frame option".