Notes

Beyond Good & Evil holds a somewhat bizarre position in gaming.
Released on every console that mattered, as well as the PC (to which, without a gamepad, it is rather unsuited), it quite successfully bombed on every one of them. This is in spite of critical praise from virtually all quarters: all the PC and console magazines rated it highly, and both Eurogamer and PC Zone published articles relatively recently re-encouraging people to play it. (Eurogamer's Xbox Cult Classics, go search for it: there's some good stuff there).
However, the game failed to sell. Beyond Good & Evil (BGE) seemed too esoteric, too bizarre. Much was made of Michel Ancel being behind it, much as Tim Schafer's Psychonauts was seen as his artistic work. Neither sold. Both are brilliant. Why?
Much of this is down to perception. True, few people buy Independent Developer-type games, but neither is a Darwinia. Of the two, BGE is by far the most accessible. But the appearance of perceived esotericism did in for it.
Not that Psychonauts is inaccessible, but it is intentionally, brilliantly, wierd. The wierdness is what makes it funny.
BGE isn't wierd. Not really. It's just brilliant.
It's got Zelda-esque brilliance.
Hyelian Class
The comparison is more than a fecetious one. On a structural level, BGE copies Zelda almost directly, with central Hubs to the world, a main town, where shops can be frequented and people interacted with, to dungeons providing the "quest" gameplay. Hell, even the controls are extremely similar.
And yet, the comparison is just as apt in another way. The storytelling and the pacing are sublime. And not-at-all esoteric. The plot is highly focussed, and (again, much like Zelda) has a strong emotional focus. There's one or two moments that are genuinely moving. Jade, unlike Link and the inhabitants of Hyrule, may talk, but the voice acting is superb throughout.
You care about the characters. That's another triumph.
BGE is significantly shorter than most Zelda games: with roughly four or five dungeons and lots of hub world activities, it's not without stuff to do, but it is a roughly 12 hour game to complete. That makes for a remarkably tight story, and actually benefits from not being over-long.
What most people miss with the Zelda comparison is that the wonderful sense of controlled freedom present in Zelda is present here. As is the wonderful accessiblity of the game. Anyone can enjoy Zelda, despite the fantasy setting, because the game is deep, involving, and - crucially - fun. BGE knows that, and hits all the right marks.
Something Unique
This game is unique, though, in its setting, and some of its characters. The game looks fantastic. Not even for its age, since the art style has the same kind of cartoony charm as Team Fortress 2. It doesn't matter that its a little old now: play it. It still feels fitting.
The world is actually relatively small, but manages to feel enormously free due to the use of a hovercraft, and, later, a space-ship, which you can use to blast into orbit. The go-anywhere do-anything attitude is wonderfully immersive, yet you always have a strong knowledge of what you need to be doing next.
The Hovercraft sections are a joy - controlling it is easy and feels natural very quickly, it never lets itself get bogged down by failing to provide you with a destination, or to restrict your activities either. The hovercraft race sequences are worth a mention all by themselves - they're far better than Knights of the Old Republic's speeder races, and brilliantly diversionary.
Also unique is the nature of the game - to an extent. Jade, your main character, is a reporter: a photographer. This informs your goals: you need to photograph evidence in the "dungeons," rather than find items or kill bosses.
Also, you have a constant mission to photograph all the many life-forms, from sentient species to flies, in the world (although to a large degree this task is non-essential past the first section).
The latter is surprisingly involving, and also brilliantly magical - waiting for the right moment for a whale to surface to get the perfect picture is stunning - and many reviewers have acclaimed this aspect. True, it is unique and special, but giving undue attention to what is essentially a side-quest throughout the game can skew the balance of perception. The game is simply not about this. Any more than Zelda is about collecting Heart Pieces.
The other aspect is that Jade needs to be stealthy in the dungeons since she is not able to fight the guards mano-a-mano and win. But the game encourages experimentation and should you wish to kill every enemy in the game this is highly possible. Everything has a weak spot. The stealth element is fun, and used sparingly enough that the "dungeons" in the game don't ever begin to feel simply about that.
The game also tries to have its cake and eat it, like most games under the woeful generic title of "action-adventure." Whilst Jade cannot fight the large guard-type characters straight-on, all other enemies she can whack with her stick, which stands in for a sword. The combat is fine, and there is actually a fair bit of combat too, as well as a number of bosses to defeat. Generally this works fine, and (particularly with the final boss) the game plays several clever tricks to make the rather routine work-out-the-pattern boss fights feel fresh.
Conclusion
Calling this game a "cult classic" or "hidden gem" actually does it down. Playing a "cult classic," one expects a lack of polish, and dodgy elements which would have to be ignored. Beyond Good & Evil has neither. Perhaps the only inaccessible facets to it were the title (Beyond Good & Evil?! What is that? It can't have helped sales, really) and the box art, which didn't seem to know what it wanted to be. Also, the lack of a franchise (this is not Zelda 8) to give it familiarity probably contributed to lack of sales.
This game is hugely accessible, and whilst the plot is original much of the gameplay is not, and this too helps with fostering an ease of play. Think of when Banjo-Kazooie nicked Mario 64's controls: that did not make it a bad game, far from it. Likewise BGE and Zelda.
That this game is on GoG now is a fantastic, wonderful new lease of life for a sadly-overlooked but always acclaimed marvel of a game. Buy it. You owe it to yourself. It's a nearly perfect game, in every way that matters.
If only they'd thought up a more catchy title.