There is plenty of discussion on the US Volt forums, a lot of the people there claim to use 'L' mode exclusively. There are no gears to damage so it is definitely safe at any speed.
All that 'L' mode does is to change slightly when you get regenerative braking. If you are in stop-start traffic then it makes it easier to be more efficient, but if you can coast in 'D' instead of slowing and accelerating that's more efficient still. I've also heard that using 'L' mode in conjunction with cruise control holds much closer to the set speed. Some people also claim that to get the best driving experience use sport mode + 'L' so you get sharper acceleration combined with 'engine braking'.
All that 'L' mode does is to change slightly when you get regenerative braking. If you are in stop-start traffic then it makes it easier to be more efficient, but if you can coast in 'D' instead of slowing and accelerating that's more efficient still. I've also heard that using 'L' mode in conjunction with cruise control holds much closer to the set speed. Some people also claim that to get the best driving experience use sport mode + 'L' so you get sharper acceleration combined with 'engine braking'.