I had a 2006 ML350 on a two year lease - never had trouble with it.
But that was 30K miles - and the few service appointments I had made my jaw drop when I got the bill.
For those who may be wondering, let me detail some of ML350's service costs and service intervals.
All MB's alternate between Service A and Service B, which are done approximately every 13K miles (the computer gives you service reminders based on the earlier of the time or the mileage). Remember that all ML's use full synthetic oil, which allows for extended time between service intervals but obviously increases the cost when you do need it.
Both Service A and Service B consist of a bunch of inspections, which an independent station can do for you for free, thus greatly reducing the cost.
Overall, my local MB dealers constantly run a Service A special for $99 (the regular price is about $130-$150), which I've taken advantage of -- it's basically a full synthetic oil change, new wipers and a multi-point check.
Service B performed at a dealer typically runs about $250 or so (with a coupon). It is the same as Service A plus air filter, dust filter and more extensive inspections. Basically, you can save a boatload of money on this by taking your ML to an independent.
Your transmission service only has to be done once at 39K miles (not every 39K miles but just once). This costs about $300.
Your brake fluid flush gets done every 2 years and costs about $100 (about $50 at an independent).
You NEVER have to service your differential and transfer case.
I just had front and rear brake pads replaced at an MB dealer for $270 total (this is cheap, however, as the regular price is about $200 more... if you can't find this special, an independent MB specialist will be way cheaper).
Fuel filter and coolant get replaced at 143K miles or 15 years.
Spark plugs get replaced every 91K miles or 5 years.
ML's do not have timing belts, so you never have to worry about their replacement.
In other words, while the services themselves are more expensive on the ML, you don't do them as often. If people are curious about reading more, MB's website contains PDF's of owner's and maintenance manuals going back all the way to 2000. You do not have to be an owner to access them and don't even have to register.