I'd call it a guideline.
If done right, CR and EL say something fairly meaningful about the challenge of an opponent, but it is based upon certain assumptions about the capabilities of the party and can be very situation specific.
I do use it to eyeball encounters, and find it is fairly good in that context if I keep the party's capabilities in mind. Which I find fairly straightforward to do. For example, last high level game I had, there was a fighter who never used ranged weapons or, for that matter, and weapon other than his greatsword. If I wanted the other characters to shine, ranged or flying attackers or creatures with good DR vs. slashing were the order of the day.
I wouldn't use the term "I'd call it an art." Being situation specific does not make it an art, any more than the matter of fact about whether my 1 GHz PPC will run a program faster than my 1 GHz Pentium 4 is "an art". It's a matter of what program you are talking about, what it's trying to do, what OS you are using, etc.
I do not use CR to calculate XP. I call that "tedious" and "rewarding the wrong thing".