Context: Lens released 2001. (This lens has been replaced by the better "II" version.) I mostly shoot portraits and have have been using this lens for 10 years - with both film and digital.
This review written in 2017/2018. This is the only Canon lens I still have.
This lens is a good old classic. This used to be what I called the “standard issue” lens since every professional with a Canon had this lens. In 2010 it was replaced with the version II which is significantly sharper. I never bothered to replace mine since the softness did not concern me at the time. This lens has the best bokeh of any lens I have used (with the exception of the 24mm f3.5L TS II) and the bokeh is the one aspect of this old lens that is actually better than on the new version.
This is a fully-functional lens including the three critical things to me: weather sealing; auto-focus; image stabilization. All lenses should have those three things in my opinion. If you never shoot in the rain or snow then weather sealing is probably not relevant to you, but it is to me.
Downsides: softness (not the sharpest lens); and the image stabilization is loud. When shooting video with this lens with the on-camera mic (which you should never actually use anyway), you can easily hear the IS in the recording – it sounds like a marble rolling inside a tin can.
Upsides: This is a great old lens for portrait stills, or for video if you have the mic far enough away that you don’t hear the IS in the recording. If you shoot Canon, buying this lens used will likely be good value for your money at this point. The newer "II" version is better - there are two differences that I am aware of: the new one is definitely sharper; and the new one has better autofocus.
This lens produces excellent results. However, as noted above, it is not the sharpest - some people call it soft. The bokeh is the best I have seen, on par with the 24mm tilt-shift II (the bokeh is the one aspect of this lens that is actually better than the new version). This lens is great for portraits. I typically use f3.2 on this lens - I consider it useable at f3.2. The photo of the deer and of the sour cherries, below, were both taken at f3.2.
Use with tele-converters (extenders)
With a 1.4x teleconverter: pretty good.
With a 2x teleconverter: soft image in my experience. I have found that with Canon’s version III of the 2x teleconverter, I get really sharp images at f9, but not at wider apertures, so I use this teleconverter only at f9 now… which is pretty limiting. Canon’s 400mm f5.6L is probably a better option - though it would take up more room in the bag to add the 400mm lens (assuming you already have the 70-200 in your bag) vs just adding the 2x teleconverter. So, I guess it depends how often you plan to shoot at 400mm - if you want to use that often (like for example if you often take photos of birds) then the 2x teleconverter on the 70-200 is not the best option.
Ergonomics of the lens
Excellent. The location of the zoom and focus rings, and the switches, is very good. In my copy, there is just a touch of looseness in the rings.
Practical? Yes. Weather sealing, autofocus, AND image stabilization. Plus a zoom. This lens has it all! …except for sharp images. I typically shoot this lens at f3.2 to f4. For landscapes I use it at f5.6 to f8. If you look at my "Dog Portraits" page, those photos were mostly taken with this lens.