Do you remember the old Power-Glide trannys that GM used when muscle cars were just hitting the scene? I'm not old enough to remember, but I have experienced them after-the-fact. They worked fine, but if you were cruising in second gear and hit the gas causing a downshift, it might be rather abrupt, because there was only one choice for the transmission to make. In the newer trannys with more gears, there are many choices. For instance, imagine cruising in gear eight, you hit the gas ... depending on how hard and at what speed you are traveling, you could drop to seventh for a gradual acceleration, or to fourth for fast pick-up. Conversely, only having two forward gears in the Power-Glide allows for excellent reliability and it also makes them predictable, which is why they are still popular transmissions for drag racing.
Now, as far as efficiency is concerned, if you are driving slowly, the transmission can up-shift to a lower numeric gear ratio, saving gas in the process. I have never driven a car with more than six forward gears ... so I can't say what the driving experience is like, but it is certainly a market trend to increase the number of forward gears. For maximum efficiency, a manufacturer may choose a CVT with an infinite number of ratios, but some consumers don't like the driving experience with them. So I would expect to see more and more new transmissions like this ZF with eight forward gears. The more the merrier?