Despite the blatantly unfinished nature of the game, there is fun to be had in Quest 64. The battle system is unique and fun to play around with, and the stat building is tuned just right to never necessitate grinding. The music is also rather good, and due to being one of the only N64 RPGs, the game has a unique aesthetic in the genre that has its own charm.
However, the game suffers in most other areas. The dungeons are overlong, and the ones that aren't straight lines can get very confusing due to both a lack of landmarks/puzzles, or the camera confusion following a random encounter. The game's story, while containing the occasional piece of witty dialogue, barely goes past the old "save the four macguffins of the elements" plot, with only a couple interesting wrinkles near the endgame. This leaves the world of Quest 64 feeling very empty and lifeless, despite having some interesting lore buried deep down that one can ascertain from the limited exploration. And while the combat is fun at first, high level spells (which can be gained rather early on) can trivialize most fights, including the final boss.
Quest 64 is by no means a bad game, but it is a bland one. It only runs for about 10 hours (my playthrough was around six and a half), so it comes and goes quickly. I feel bad for it, due to it being rushed out the door and its sequel cancelled, but only glimmers remain of what the game was meant to be, and it should only be played by those curious about the N64's most well known RPG.
EDIT: "Most well known" might be a bit of a stretch, fair. A better way to put it might be most well known 3rd party RPG?