I am satisfied with ML320 I purchased. It doesn't pin you in the seat when you floor it, but then, I didn't buy it for that purpose (that's what the SL is for).
It's smooth enough for the highway and can get out of the way when you need it too, just not with a lot of authority. If you're looking to tow with your SUV, look towards the V8 versions.
Haven't had any problems so far with the drivetrain...also, it doesn't have many miles either.
I suspect the head gasket failures that were prone on the M103 and M104 series motors (like my 300E) were a combination of the different metals of the mating surfaces (block and head) combined with the large surface area of the L6. Thermal expansion is more of an issure because of those two factors.
The V6 and V8 have less surface area on each bank of cylinders so the head gasket threat is averted in that case.
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car