I debated quite a bit between either getting the Canon 70-300 IS USM or the Canon 70-200L f4 (non-IS). I figured that for a 1.6x crop body, without IS, even at 70mm you'd need at least 1/100s shutter speed to avoid camera shake. Likewise, at 200mm you'd need 1/320s. Now, I don't know about where you are but in cloudy days or indoor arenas, at ISO 200 and f4, you might barely get 1/150s or so. Since I exclusively shoot hand-held, that pretty much limits my zoom to about 100mm - not good enough if I'm shooting slow-moving wild life. One option is to bump up the ISO to say 800, which would afford you higher shutter speed but at the expense of image quality (more digital noise). So, for me the IS is the crucial piece of technology that would allow me to lower the shutter speed and lower ISO level enough that results in an optimized image quality. Of course, none of this would be an issue if you: 1) use a tripod/monopod, and/or 2) have another $500 to burn, then the Canon 70-200mm f4 IS USM is the way to go.
Moving on to Auto-Focus (AF). I've read that quite a bit of users are having trouble with AF in low-light. The issue is not entirely on this lens, but also on the camera body. Just like "it takes 2 to tango", it takes both a quality lens and camera body to capture decent photos. Now, I'm not a pro by any means, but I've done enough research to choose a camera body that fits my needs. Just like buying a car, you wouldn't buy a Honda Civic and expect it to have as much space as a pick-up truck to haul things around. Conversely, while you could buy a Ferrari to drive the kids to school but that would be a waste of money. So, what's my need, you might ask? Over 80% of the time, I plan to shoot in indoor arena (horse competition, sports complex, etc). And, given my budget, most of my lenses will probably have a range in f-stop (say, f4-f5.6). Given these 2 criteria, I chose the consumer-level Canon XS body, instead of XTi, XSi or T1i. The reason being is that the XS is the only camera in the consumer end that employs a cross-type AF algorithm that is optimized for lenses f5.6 or faster. So, I rarely have AF issues even in extreme low-light conditions. Love it!
So, is the lens perfect? No, not quite. For one, I wish I had more money to buy a fixed f2.8 IS USM zoom to capure indoor fast-action shots. But then again, as a budget concience user, I've learned to take opportunitisc shots of actions in transition. Just like when you throw a ball upwards at high-speed, eventually the ball will momentarily stop to make a downward transition. You just have to learn to use the lens for your application.
Well, enough said. Go buy it and you won't be disappointed. By the way, my copy has the number '4' as the 3rd-digit on the SN#, so no zoom creep nor the vertical image quality problem as part of the Canon recall notice. Good luck!