Quote Originally Posted by Unoriginal View Post
In particular, Beholders tend to have big, quirky, hammy personalities, as well as powerful but alien minds, and both should impact the fight quite a lot.
Completely agree. The quintessential Beholder fight is a crazy video game boss-fight with platforms and moving parts and death ray beams flying in all directions, except you're up against a Dr Who villain.

Mechanically, there are two issues to consider. The first is the case with any "boss" monster in 5E with legendary actions...those 180 hit points don't go so far when the whole party is single-target bashing on it. Canon fodder minions and/or interesting terrain make or break the encounter.

The second issue is one you've already alluded to...the save-or-suck nature of the Beholder's attacks. They take the player out of the fight for an extended period, which might make the fight a slog for a particular player (which is bad, and should be avoided), but amps up the pressure on the party as a whole. Most of them are not IMMEDIATE sucks, they are "sucks for a round or two", or "sucks if you're low on HP". Death and Disintegrate are scary if they reduce you to 0...so players should be well aware of the consequences and in-combat healing actually becomes relevant. Fear, Charm, Slow and Paralyze all let the victim save each round, so they aren't so bad (paralyzing a player surrounded by cannon-fodder minions is a rough trick, though, even for a round, so be mindful of that). Telekinesis can be fun, until you've telekinetically held the barbarian 30 feet in the air and they can't act until the Beholder is incapacitated...I houserule this to be a concentration effect, so beating on it might make it drop a held target. Petrification gives you a round to react and a second chance to save; you'll still want to make sure the party has some means of unpetrifying someone.

I've seen two beholder fights and this was the deciding factor between the one that was a blast, and the one that was less so. In the first, the party had a good idea of what the creature could do, and the means to counter some of it. Lots of actions spent healing, slapping someone awake, buffing a save or etc. The whole time the Beholder was making creepy, cryptic non-sequitur statements about dreams.
Dynamic, and strategic, and engaging. The other was just a slugfest, where they ran in and beat on it, knocking off one player at a time, but still doing enough damage that it was dead on round 2. Boring, and half the party barely got to act.

TL/DR; it can go either way, depending on how you play it, and what the players are into, strategically.