I can tell you about the GLC43 AMG (same chassis, wheels, box, engine, most of the interior) if that helps (2017 model, not the coupe’, the normal one)
MPG? well… it’s got 5 modes from eco to sport ++.. eco you’ll get about 33mpg mix of lanes/A-road/M’way, switch to the other modes and it drops accordingly. Sport++ is harsh riding, stiff and rattles come through the bodying the cabin and it squeaks. Not sure if the build quality is pants but it doesn’t feel screwed together very well.. It’s bonkers fast and immediate thats for sure, and the box changes are instant and sublime. Eco it’s just a big lolloping SUV.. comfortable, but he seats are a bit tightly bolstered so if you are fat you’ll struggle to get comfy. All the rest of the stuff is typical Merc, nice to touch, works well and the infotainment stuff is bonkers clear and loud..
The downsides are the wheels are made of plastic and crumble at the slightest rut or pothole, two nearside wheels have needed replacement due to buckling and the tyres have been ruined accordingly. Not sure why Merc make the alloys so thin, but they are and it’s a typical fault because the dealers always have them in stock.
Rear seats are hard, bench like and not very comfortable.
Boot space is really good, with the seats down 3 bikes can go in it with the wheels on pilled on top of each other.
The rest is just Merc.
Its’ not mine BTW, it’s my best mates and we use it a lot for razzing to-from work. I’ll be honest, but this is just me, I would spend my money elsewhere.
HTHs.