I have the B 200 Turbo with CVT and I find it is very smooth and always in the right gear. Then again, I leave it in drive, on the comfort setting and just drive it. I do not shift gears or second guess the transmission control module.
My transmission exhibits none of the bad characteristics that Mike has described. There is no hunting lurching etc. On a downhill run, it will hold the gear if your foot is off the gas- but, if you give it just a hint of gas, it will continue to shift up and coast in the higher range. The transmission module also memorizes your driving style and adapts to your driving technique.
Some prefer a manual transmission and that is OK too - that is why MB offer both and call it an option.
For decades, the Germans eschewed air conditionning and automatic transmissions: on the basis that real men don't sweat and real men can shift their own gears. Today, air is standard and automatic is in the majority.
Please remember that all of the advice posted here is probably worth exactly what you paid for it.
