Never drove ML but the X5 is smooth and nice. Cornering almost like a sports car :P The standard power steering sucks tho. I am getting buffed up since driving it... kinda tight. Should have bought the active steering option.

About refilling the tank... isn't there any staff at the petrol stations in the US?? I am sure for some tip they gonna fill it up.
If you gonna happen to do it yourself, it usually wont do any mess.
Make sure no overfill. The fuel is colder under the ground in the container, then its temperature increases once inside your tank. For that its volume will increase also, so if you overfill right before reaching home or you park the car right after refilling, the excess diesel may leak at the fuel inlet.

But it does not happen often.

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X5 3.0d - Space Grey , Beige Leather, Burl Walnut Trim, SatNav, Rear Camera, Sunroof, Light Package, 19" 212 Wheels, Tint, Power Fold-in Mirrors. AUDIO: Morel Elate 6 Audiophile Speaker System with Genesis Dual Mono Amp and Rockford Fosgate 3Sixty.2 DSP