Beholder probably wouldn't be fun at all. If you assume they're anything like presented in the MM or LoM, you'd be more or less compelled to try and murder the rest of us for not looking like you do. Even without that, you'd be effectively reduced to our door warden/base guard for most activity time, since you have no way to effectively disguise yourself outside a full-on Polymorph effect (which would remove all the goodies of being a Beholder), and your ability to work through minions would be limited because your best control effect is a Charm Monster.
If you want to be a caster, what about a rakshasa? They've got shapeshifting and innate sorcerer casting, and can bake their LA into their HD and let you take some class or PrC levels .