This image was taken at 720mm with EVF on.

There are four IS modes in HS10:

(IS Mode 1) Continuous IS: IS is always on all the time before and after the shutter release button is pressed.
(IS Mode 2) Shooting only IS: IS is only on after the shutter release button is pressed.
(IS Mode 3) Continuous + digital IS: there is no explanation of how "digital IS" works in the manual. From my observation, the camera takes 2 consecutive shots and combine the two images into one shot (that's why it is called "digital"), with continuous IS on.
(IS Mode 4) Shooting only + digital IS: same principle as above, but with shooting IS on.

This image shows 100% crops from original photos, with shutter speed ranging from 1/800s to 1/13s. Since high ISO had to be used for getting a fast shutter speed, more noise/noise reduction could be noted in the images taken at 1/800s, thus resulting in more crispy images.

I can get the following conclusion after the test:
(1) Handheld condition could only give you 1 stop slower shutter speed than required, while IS mode 1 and 2 can give you 3 stops slower shutter speed than required.
(2) Since digital IS takes 2 shots, double images would be resulted in some shots. When shutter speed is within 3 stops slower, I cannot see any advantage of using IS Mode 3 and 4, but when shutter speed becomes further slower, IS mode 3 and 4 could produce "sharper" result than IS Mode 1 and 2.

My advice:
(1) If shutter speed is faster than the "safe shutter speed", switch off IS.
(2) If shutter speed is slower than the "safe shutter speed", but faster than 3 stops slower, use IS mode 1 or 2. Both of them give you similar results.
(3) If shutter speed is slower than 3 stops from the "safe shutter speed", use IS mode 3 or 4.
(4) For sure is that if you have tripod, always switch off IS to get the best result.
(5) After all, if you only print photos in small size (say 4R), I would conclude that all 4 IS modes could give you good results.
(6) Small tips to prevent handshake: high speed shooting mode, e.g. 7fps, could reduce handshake. Try it and tell me the result!