I normally keep new cars for about 3 years, and I'm pushing 3.5 on my Magnum RT. However, nothing new I test drive measures up to this car. First, the Mercedes E-class chassis means the car has an excellent combination of performance and comfort (not a typical muscle car). The engine is as strong now as the day I bought it, with surprising torque that makes it feel faster than smaller V8s (Audi S4, Mustang etc.). Has had no problems other than poor stock tires and rotors, and feels solidly built. I'd highly recommend it as a used car since values have dropped - they will likely depreciate little if you buy one now. If you test drive one, you will buy it.
That's easy - the engine. Perfect combination of American muscle and European ride quality.
I miss having a manual transmission, but the Mercedes-sourced auto is amazing. The SatNav is useless - a $1,500 option and I end up using a Garmin.