I used this in Thailand, along with Deet 100. The place was inundated by mozzies but i only got 1 or 2 small bites.
I didn't notice any huge difference between 100 and 55. 100 is very strong and probably a bit bad for the skin so I would probably only buy it again for a tropical kind of place.
I used 55 again this spring in the south of france where there were loads of mozzies and I didnt get a single bite.
Works well against Scottish midges too. Better than Avon skin-so-soft which stops them biting but doesnt stop them landing all over you. Deet is better for keeping them at bay.