I have both the original 5D and the mkIII and I am still happy to use the original one, although the mkIII is my main camera of course.
Used within its comfort zone, the original 5D produces great images (I am still getting images taken by mine accepted for national exhibitions) and you would struggle to tell them apart from the mkIII images except by resolution. For smaller prints (up to say 24x16) or web use, you would find the images from the classic equal to those from the mkIII.
However, the key part there is "within its comfort zone". If you need higher ISOs then you will see a difference (or be unable to shoot at all, as it only goes to 3200) so light needs to be good. The AF system is great on centre point, particularly with the assist points engaged, I find that it tracks very well. However the outer points are only usable in good light and even then can be dodgy if the lines of contrast are orientated the wrong way for the AF point. When conditions are good you will see little IQ difference between the two, in the end result, however the rest of the time the mkIII will be clearly ahead and often I have got great results from the mkIII in situations where the classic couldn't even make an attempt.
It depends what you shoot really. If you are primarily a studio shooter, and don't want huge prints, then the classic is still a great choice as you control the conditions and can keep them where it works best. There is also a certain indefinable tonal quality to a 5D image, with skin tones in particular, that is very pleasing to many people, even in preference to the mkIII.
If I could only have one body it would be the mkIII, however I would be sad to be without the classic as it still produces great results in the right conditions.