here is the conclusion: I like the lens...a lot
* Close up performance is nice when not wanting to change lenses (I took some surprisingly good butterfly shots with it), but it in no way reaches the performance of the 100L macro lens. Another issue with the 70-300L is that it is missing the focus limiting switch of the 70-200L and 100L (thanks, Canon!), so if the af misses it takes a lot of time.
* The IS on the 70-300L is really good (at least 2ev) and the lock is instant and nearly not noticeable - but there's a catch since it still needs a little time to be most effective. So for quick point & shoot it's nice to have a stable viewfinder and being able to set the af precisely, but in my experience waiting for a second is a good idea.
A focus limiter would have been nice. No, it isn't close to the 100L's performance, but that is not surprising. I haven't tried it with extension tubes, but I suspect it would do pretty well.
Bryan Carnethan at the Digital Picture rated the IS at actually better than 4ev, but testing IS is pretty subject. I am very stable, myself. This sample was taken (also yesterday) with the extender attached (420mm) at 1/15th. That's pretty impressive. I suspect that with a static subject I could pull off a 1/5th at 300mm. I haven't actually experienced the lag that you describe for the IS to kick in, but I am rarely a speed shooter so it may be possible.