I find it interesting how many people sang the praises of the mkI for many years, calling it one of Canon's great lenses...then suddenly it's a dog when the mkII comes out. Did the lens change? Or our perception of it? Obviously the latter. Certainly the mkII is the better lens, all sources indicate to. The question is, whether you have to spend the additional money for it, or would the mkI or Tamaron work satisfactorily for your needs? Only you can decide. I have the mkI, I find it's satisfactory for most of what I need it for. I'm happy with images from 100mm to 200mm, at 70mm I seem to get less critical sharpness, though not unusable. Fortunately, I tend to use this lens more at the upper ranges, so this isn't a dire issue for me. I do like to stop down to f/3.5 when possible, but I don't shy from f/2.8 if needed, and can get satisfactory results. On portraits (what I mainly use it for) I get detail in eyelashes and such, so it serves it's function, and the bokeh is a wonderful creamy texture (I've heard the mkII is a bit more harsh in that feature). Would I like to have a mkII? Sure, sounds like it would be nice. Do I feel I NEED a mkII? Not necessarily, I'm able to produce completely acceptable and sellable (the last being the most important to me as I'm a full time professional) images with the mkI, so as a business decision it's better for me to just keep the mkI.
Should you get a mkI? If you want the most critically sharp lens, and money isn't an issue, then probably not. If you want a reasonably good performing lens, one that was widely accepted as a quality lens for over a decade when it was a current model, and can find a good copy at a decent price, it may meet your needs. That's for you to decide, don't let others opinions with no perspective on your needs be the only factor in your decision. Calculate you needs and make an educated decision from there.
