Paul - I have an '07 GL450. Have owned it for about 7 months or so? Had about 50K on the clock when I bought it, came Star Certified from dealer.
It's a fantastic vehicle overall, we love it. Tows our boat effortlessly, lots of space for the kids and gear. As MB DOC stated, it's a thirsty beast, but this is to be expected and the mileage is actually quite good considering the vehicle's size and weight and 330+ hp engine. It is an absolute tank in the winter, it has handled several blizzard highway drives already and it didn't slip or skid once (I have winter tires on it now, Blizzak DM-V1).
Issues we have had have been minor ones - coin tray won't stay closed, fussy powered liftgate not always closing (I think the ML's had this as well, apparently its due to the oversensitive sensors), one LCD screen went out, it was a fibre-optic cabling issue. All warranty covered.
I have read about frequent CEL's from other owners related to magnetic camshaft solenoids. Fortunately, it is a cheap part and easy replacement. Not a big deal.
Our GL is actually at the dealer right now for the only major item thus far - a camshaft sprocket replacement. Glad the warranty is covering this one.
Overall though, for a first model year, it's been solid. Previous records don't show anything drastic. When we bought it, the dealer had the tranny fluid replaced for us, which was nice.
I can see though that this will likely be an expensive vehicle to maintain. It is very complex and fully loaded with every conceivable option. We have a heated steering wheel, powered folding rear seats, entertainment units on headrests, nav, bluetooth and iPod integration, air suspension, yada, yada. It's more like flying a plane than driving a car to be honest.
So...long term? I am thinking many of these little motorized and electronic gizmos are going to break and won't be cheap to replace. Not sure how much DIY I'll be able to do. Air suspension is amazing (love the lowering/raising feature, handy for hitching) but they are not cheap to replace or fix. Again, you pay for the luxury. So I think this is merely something one must factor into owning a vehicle like this.
If you are looking for a cheap SUV, perhaps look elsewhere. If you want to drive an amazing luxury SUV, go for the GL, but she won't come cheap.
Good luck, sounds like you are doomed anyway.
My wife also loved this car from the outset though the sticker price was a tad daunting.
C.