Not going to lie, I kind of
hate Ben Solo from a meta-perspective; he’s the painfully conventional and shallow story element demoting Finn from male lead and the ethereal “What if?” they use to justify screwing up Rey’s character by making her romantically interested in him... which makes their interactions basically a shallow abusive romance where Rey has no reason to be interested in the guy repeatedly hurting her... but expresses such anyway.
But the general consensus here is correct; Driver is fantastic, but the material is less exploitive if his skills, and the character goes through a rushed arc:
His arc felt very hamfisted and unearned. He was still great in the role, but I don't think it worked as much as it should have.
His body language after the shift in characterization between Kylo and Ben is great, especially the shrug when he gets the lightsaber... but the shift isn’t the result of a real character arc, but instead basically waving a magic wand at him. And since the character wasn’t really written that dynamically as a villain in TLJ or in the first part of TROS to really set up the change, they have to just pull Han back, do the same TFA scene from the first film, and just have him choose differently this time.
The result is that he’s not really well characterized as Kylo... and doesn’t get any real time or dialogue to form Ben. “Ow...” is hilarious... but that’s all he ever says.
Driver could have played an iconically interesting villain, but the character was waylaid by competing visions and objectives: Abrams set him up for a greater descent into villainy and a rise in effectiveness as an antagonist... for the
next film, but Johnson wanted him to instead be a somewhat static Byronic hero who’s heavily internalized struggle was supposed to suffice for conflict, and who’s only real accomplishment was killing Snoke, but in a way that didn’t help him get more intimidating... and never got a win back over Rey.
Meanwhile, everyone presumed that Ben Solo’s redemption would be the character”s endpoint and was highly valued... but wasn’t set up in any real way. So, Abrams throws in a quick but unexplored explanation about Ben being some kind of Force Schizophrenic thanks to Palpatine, and just conjures up Ben - at the low, low cost of both his parents’ lives. And then, whether because they just have that much faith in Driver, or because they realize thatntrying to form Ben right now risks making decisions that might alienate or sadden the audience.
To me, the best portrayal Driver gave for the character is still TFA, but that’s just because that film had enough focus to use his talents perfectly there for a role that promised greater things to come. TROS tries to squeeze in far more to show his range, but it’s not so much a story or arc as an outline he shoots through.