My wife and I love our X5 but with two kids (3 and 4)we were busting at the seams. We drove a Honda minivan first and then the R350 on a whim -- in our view, the R offers 85% of minivan functionality but feels 250% more refined. So, we snaped one up on an manufacturer subsidized lease, (monthly pmts less than a Honda!) and immediately packed up the family for a trip to the NC mountains where we drove 40 miles up and down hills with switchbacks that would have made me sweat even in my BMW 330i. The R inspired confidence and never lost composure even when late braking on decreasing radius turns going down a steep grade! To me, this represents a level of active safety that few can match.
Mileage better than expected, confidence inspiring brakes, and incredible road manners, even at the limit. Enormous interior, and the quality / experience you expect from MB. I can't believe such a large vehicle handles so well.
Long wheelbase - takes a little getting used to especially in tight quarters. Could use some more power, but I am used to high performance vehicles so it is probably OK for most.