I recently purchased a 1999 S320 with very low miles. I shared the concern about the car being underpowered. In fact, I came out of an six year-old M3 I had babied since new - but family circumstances were forcing me into a larger car, and at 6'4" the old S-class is the most comfortable car for me (and any rear seat passengers behind me). The transmission ratios seem to make the S320 perform nearly the same as an S420 - though it lacks the serious punch of an S500. To date, I have found the acceleration adequate and the car is simply awesome on the highway. Plus I love the exhaust note and smoothness of a straight six.

If you really need to win stoplight derbies, and are willing to pay a price premium for it, the S320 is not the car for you - get the S500.

To put things in perspective, not too many years ago a 0-60 time in the low to mid 8s was pretty good for a midsize sedan, much less one that weighed over two tons.