So, now I've played it a fair bit more. Well past the point I got to last time I played it - I'm up to the Miriam Stilshrine, about 20 hours in.
I… don't think it's very good.
It's very, very repetitive. Every area is basically the same. Big mazes full of monsters wandering around and you walk about and kill them then you go to the next area. Open areas are like this. Dungeon areas are like this. There are no side-quests apart from "Hunts" which are basically going to one of the big areas and killing a boss that now spawns there. Occasionally you'll get a plot sequence but then you're back to those big areas full of monsters. There's not even any attempt to make the areas particularly believable - OK so this mine might be full of bats and that sounds plausible but oh, what's this? A T Rex. Of course it is.
Yes I get that this is basically a single-player MMO but the problem is it's based on a very specific style of MMO - essentially the original Everquest style of grindy, open MMO - and in fact on a very specific game which is of course Final Fantasy XI. And as a single-player game it's repetitive as hell. The Gambit system is very clever but it also means that for everything other than boss fights (where you might actually need to mix it up a bit) you're barely playing the game, simply walking into enemy after enemy and letting the battles play themselves.
In particular it suffers badly in comparison to Xenoblade Chronicles. Now that's not an entirely fair comparison as there are six years between the games but technically they don't feel a million miles apart (PS2 vs Wii) and they're both based on an MMO style of gameplay so it's impossible not to make that comparison. But Xenoblade is massively more engaging - every area in that game is full of side quests, so you've always got plenty to do, and while your team is also largely automated, you do have full control over one character so you're at least busy in the battles. The characters in Xenoblade are always talking to each other, too, shouting encouragement and generally bantering with each other both in and out of battle, which makes them feel much more real, whereas FFXII's team fight battle after battle in grim silence and only ever actually converse in the very occasional cut scenes (all of which exist for plot reasons, never simply for character-building).
FFXII is just dull. Most of the actual storyline happens elsewhere, to other people, and your team only ever reacts to events; meanwhile you trudge relentlessly through cut-and-paste brown and grey corridors fighting the same creatures with slightly different names and colours, wondering when anything's actually going to happen. The ability to run the game at 2x or 4x normal speed cuts down on the drudgery a little but also kind of shows how the game was so fundamentally flawed in the first place.
I do like the voice acting and it does some interesting things with regional accents (the strange European accents of the frankly slightly embarrassing bunny girls is a particular standout) but even then it's oddly inconsistent - why is the entire Rabanastran aristocracy British, apart from Princess Ashe who's American?
I'm perplexed by its enduring popularity.