There's a few things you need to know to do it well.
1. EE-S screen in a 5d - makes focus pop WAAAAY better.
2. Select AF point (I always use center single one) and half hold shutter button whilst bringing the image into focus, with the center point over where you want to be in focus. When it comes into focus the focus confirmation light will light up green in the VF and you can make the camera "beep" as well, and the AF point will flash red. Alternatively you can use all the focussing points and they will light up in succession as you focus, with the focus being on the point that lights up at the time.
3. With just a little bit of practice you can do it with just focussing and the ee-s screen though you can't expect it to be 100% precise in critical focus.
It sounds troublesome, but it's only because I've tried to explain it clearly and in detail. Overall I find focusing the 35 ZE on my 5d to be more accurate than most AF lenses, though they take about twice as long to attain focus - which isn't very long. Certainly no longer than a manual focus film SLR.
I'd say I could take 50 close and medium distance shots with the 35 distagon wide open and get 45 perfectly in focus
@ f2: