It's about $3 more than I think it's worth. But I've still splurged and gotten one of their 'limited' ones which was like $17+tax? That one was NOT worth the money (it was the D23 exclusive to expo attendees milkshake and yes I'm stupid lol). Plus you also have to pre-tip your server strangely as you pay inside before you are seated. So pro tip: get your shake to go and skip the tip for someone who just drops your food off at your table and doesn't actually take your order or do much else. (Sorry BT employees, but I don't like this forced pre-tip before I get any service... IF I get any service... system. Handing me food at my table is not a step up in service from handing it to me at the window.)
But as a non-milkshake fan - I'm much more of an ice cream sundae fan - I think their milkshakes are decent. But the reason I enjoy them is the toppings (cake, brownie, cookies, etc.). The milkshake is actually not as big as you'd think for the price. I'm not a big person and I can finish it in one sitting. However, since you essentially get two desserts for each of these if you consider the topping, I'm willing to periodically splurge to enjoy the silliness of having it.
I also think their fried mozz is terrific. And i also think it's $3 more than it should be. Yet when I can afford to do so, I splurge on that, too.
So: TL DR: If your priority is a great milkshake, skip it. If you want to splurge on something fun-looking at least once that's two desserts in one on a hot day, then I definitely recommend.