As implemented, it's beyond terrible. The only way I know of to disable it is to turn off mobile data (or disable mobile data for google services / maps). Maybe that's changed semi-recently, I dunno.

I can understand the utility of a feature that can dynamically adjust your directions so that you're always on the fastest route, but what Google has implemented has serious and dangerous flaws. The fact that the change in route is automatic and without my consent unless I interact with my phone in a set interval of time WHILE DRIVING is ridiculous, and once it switches over, there's no quick and easy way to undo that.

Additionally, it doesn't seem to understand that I don't care if the toll road will save me 3 minutes on this trip, I don't want to pay a toll or deal with the inevitable billing screw up. I don't care if making a dozen turns through unfamiliar urban/suburban side streets will questionably save me 5 minutes, I'd prefer the relative safety and predictability of a straight path down the interstate. I also don't want to drive down a winding, unlit rural highway after dark during deer season just because it's a few minutes faster than if I stayed on the well-lit road with a lot more vehicle activity.

And, that's just the start of my Google Maps rant.