I seem to have started a discussion here. For what it is worth I am the heretic (see many of my other posts here and elsewhere) who simply does not like IS. I, personally, find it to be a virtually useless feature - with one exception! Namely the Canon 100-400 Mk2 - I simply cannot hold this lens steady below 1/400 sec! When I had a 400 F2.8, later 600 F4 and now 800 F5.6 I have no use for IS, but that 100-400 Mk2.............
In my previous post I mentioned the Canon Rep regarding the fact that IS can interfere with AF which he agreed with. Well he was a wee bit sceptical, to say the least, regarding my criticisms of IS with long lenses. He was trying to tell me that Canon have the very best stabilisers going (which I agree with) but I contested that they were still more of a problem than an asset (note the exception of the 100-400 Mk2). Unfortunately we didn't have any really long lenses at hand but after a couple of minutes "tuition" I had him getting as sharp, if not sharper, shots at 1/160 with a 500 F4 L hand held with IS off than with IS on - he didn't want to talk to me after that!
Use what suits you and your needs best! With small, light, but long lenses like the 100-400 IS can be handy - I use it on mine! However for shorter lenses and the big heavy primes - I have yet to find a use for IS on modern cameras. Not having a go at anyone or any manufacturer but the greatest increase in the sharpness of my shots came about when I turned IS off. If it doesn't work for you then no big deal just turn IS back on. Try it and see