Personally, I don't understand these complaints. In good light I have no problem focusing this or any other Canon lens, nor did I have any problems in this regard with the 5D and the old 24T/S. I estimate that I have to revert to Live View for focusing about 5% of the time and that is usually because of extremely low light, say at twilight or in cases where I am jammed up against a door frame and can't get behind the camera. I do have to adjust the diopter occasionally. I have extremely good distance vision, 20/12, and poor close up vision (I have to use reading glasses to see the live view in focus). I don't see how that relates to focusing through the viewfinder though, but maybe it does.
I've come to rely on Live View almost 100% of the time when shooting architecture. I prefer it too for the process, it slows me down a bit and is makes me more methodical (read-> reminds me of MF & LF days.)
I can't say that I ever really had a problem with focus on the 24 t/s or the 45 t/s. It was more the new 17 t/s that drove me to use Live View. One might think,
eh, 17mm @ f/11 or f/16, it won't matter where I focus... but it does.
All said, once I saw that I can have 100% accuracy of putting the focus point where I want it, it's hard to work any other way.
Was my focus off with the 24 or the 45? I'd shoot tethered and would check the files to make sure the DoF was falling exactly as I wanted. Once in a while I would need to tweak the focus point to move the DoF range backward or forward. Using Live View has negated the need to futz with the focus in that way.
Maybe the difference here is like your camera never getting dusty... :-)
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