Yeah, the background blurring is wonderful with these lenses. Those shots were with the deer quite close, and the backgrounds pretty far. Like 15 feet and 60 feet or somewhere around there, so the blurring was pretty extreme (love it, though!)
Here is another, day prior to those above, with less background blurring:
Still pretty amazing stuff. These lenses are worth the cost if you want the best IQ.
The first shot there actually isn't as sharp as the rest. I think the IS group kicked in or hadn't settled when the shutter opened, so there is just a touch of some motion blur. Sometimes that happens, particularly if you bump the lens or something as you press the shutter...I think the jarring increases the IS settle time. I'm always in mode 3, which only activates the IS group after the shutter button has been pressed. That may be why it happens...I suspect if you were using mode 1 or 2 where IS is active when you meter or focus, it should all be settled by the time you hit the shutter button.
These shots were also at 1/50th shutter, which is pretty close to the 4-stop IS limit, so the chances of something like that causing a problem are a little higher. I think 1/40th would be a full four stops, at least for a full-frame camera. If you were using APS-C, then 1/60th would be a full four stops. I've actually managed to get sharp handheld results as low as 1/15th shutter, which is a little over 5 stops of hand-holdability.
When publishing online, you can see it generally doesn't matter. Output magnification just isn't high enough. It would only be an issue when printing large. (Although, that's usually what I do...my base print size is 24x36" canvas gallery wrap, and I think the amount of blur in that first shot at full size would be visible.)