Just like the Ixus 120 and 130, the function buttons on the back of this camera are tiny and completely flat to the surface. Underwater (meaning inside the housing) this is not an issue, but I will always find such streamlined cameras a bit hard for old eyes and big fingers.
Other technical details follow the same theme as the Ixus 130, so please read that review. Perhaps the quality on the 100 is slightly lower than the newer models but only perhaps and mostly, for fun photos and nice videos, it is really very good. The lens is only a 5.9-17.9 mm so not quite as wide ranging as the newer versions but you can add the Inon wide angle and macro lenses.
The dedicated underwater housing for this camera is just how I like them, meaning small enough to slip into my BC pocket. It will take a flash and other extras but that isn't what I am looking for.