For pen-and-paper D&D enthusiasts only.
Eye of the Beholder evidently takes place underneath the city of Waterdeep, in the Forgotten Realms campaign setting. Not that you'd know this from the box or title, and aside from the few references to Waterdeep landmarks that only a hardcore fan of the setting would recognize, there's little to suggest that this game takes place in anything other than the default quasi-Greyhawk setting of generic D&D.
There is next to no story; the opening cinematic consists entirely of static scrolling images, with no dialogue in text or voiceover. In most action games, this would be irritating but not unthinkable; but since Dungeons & Dragons is largely about storytelling (and this game is ostensibly an RPG), this is inexcusable, even for such a simple game.
The graphics are something one would expect from the Game Boy Color; they manage to be both bland and cartoony at the same time, while lacking any distinction between most types of wall surfaces. The primitive "3D" first-person view is antiquated and far from easy on the eyes; unfortunately, this isn't helped by the fact that battles take place entirely in a tiny isometric view.
Said battles use 'miniatures' that are so tiny as to be stick figures; they only have two frames of animation, which consist of wiggling around. The monsters are even more generic; so far, I've only seen "goblin" and "skeleton". This lack of visual variety would be excusable if there was some kind of identification for the various monsters; there is none, before, during or after the battle. This further serves to genericize the game's setting.
If it seems like I keep comparing the game to tabletop D&D, that's because the game requires familiarity with it. It uses almost the full 3rd Edition ruleset, both for combat and for skill checks outside of combat, and you'd have to be familiar with them to play the game well. Unfortunately, D&D players will strongly dislike the limited options; there are only the four main character classes (cleric, fighter, thief and wizard), and no statistics are displayed for any of the weapons. Moving characters around in combat is a chore, due to having to press diagonally on the + Control Pad for every movement, as well as the fact that characters can't be moved through diagonal gaps between each other; neither of these are helped by the fact that most of the game's combat takes place in narrow hallways. Inventory management is also a huge hassle, but unfortunately it can't be explained without demonstrating it.
Overall, there are certainly better RPGs out there for the GBA, but someone who plays D&D and is looking for some on-the-go action can bump up the grade by one.