I had the v1 70-200 that you had and bought the v2 sometime after it came out. I believe they rated the IS as being at least one stop (maybe 2) better than the original, along with improved optics. Canon was not lying. The 70-200/2.8 L IS II was impressing me as soon as I started shooting with it. MUCH better images than the v1. I was nailing tack sharp images handheld at shutter speeds that should have precluded those results unless the camera was on a tripod.
Of course, this is on a DSLR. Are you asking in the context of video or stills? I have and sometimes still use it on my C300 and the IS can help some doing handheld. Just remember to turn it off when shooting on stix. Otherwise you'll be chasing the image around. Unless you want a weird, almost handheld, floaty look.