The Beholder represents some of the strong failings of the combat system in d20. Mainly that it's a creature where facing is important, yet there aren't really any rules for facing within the system.
This is also a creature that absolutely has to be played using miniatures and a grid. Otherwise, it's just impossible to adjudicate where the beholder is, and what rays he can use, and where his cone is pointing to at any given time.
The other thing that's important to remember about the beholder that isn't intuitively obvious (at least not to me initially) is that the Beholder can't shoot anyone with it's rays in front since the anti-magic cone negates the ray. Unless you rule that the Beholder can close it's central eye at will, and I'd probably argue with that one if the DM tried to do that to me. Allowing that would also bump the CR up quite a bit.
The other question that arises with this creature is can the beholder turn as a free action? In my last campaign, the party was fighting two beholders, and during one round, I had the beholder facing a particular direction and the other tried using his ray. The player pointed out that the other beholder was in an anti-magic zone and couldn't. So, I simply rotated the first beholder 90 degrees to rectify the situation. I ruled that anyone could simply change their facing as a free action. If anyone could do that easily, a Beholder could.
The other annoying aspect of the Beholder is that you have to keep track of which rays are located where. If certain rays are on the right side of the creature, it's just not fair to the players if they happen to be on the left side of the creature to get attacked by a ray that's been attacking people on the right side. So, you have to keep track of which ray is where. And 10 rays can't be divided by 4 facings, so that's another annoying aspect of this creature.
So, in short, coming from someone who has ran multiple Beholder encounters in our last campaign, Beholders are dangerous, and complicated creatures.