I have a UK spec SL280. Physically it's the same as the SL320 and SL500 but obviously slower.
Here in the UK, chronic city congestion combined with strict enforcement of speed limits means performance is becoming less of an issue when buying a car. The £8,000 ($12,000+) saving over the SL320 seems worth taking. See www.mercedes-benz.co.uk
for more details.