If you think the R350 is slow off the line, try an R320. The low end torque (398 lb-ft) is addictive.
I was actually hoping that Mercedes would build a diesel wagon for US sale, but the closest I could get was the R320.
Also it's not a vehicle you see every day and it is very roomy for a larger person such as me. I did not fit well in the previous E-class, and the R class seems a bit roomier than even the current S-class.
They are definitely a niche item... a Mercedes for minivan customers.
Odd how VW re-badged the Dodge Caravan in the form of the Routan but MB started with a clean sheet of paper design.
As for the suggestion that the R-class is like the Pacifica; I would offer that the Pacifica is smaller and harder to get into... I definitely do not fit into a Pacifica. So, yes the R-class has a bigger door.
One of the car magazines claimed that the R lowered people's pulse by a point or two....
Also on Motorweek on Speed they rated the R320 the 6th meanest vehicle in terms of some environmental rating, with a 10 mpg Lamborghini in first place

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic