the difference is not so obvious
How is the difference?
After having used this lens for a few months and seeing the results it produces, it is easy to see why so many photographers wouldn't be without it.The build is very solid.  Ultrasonic motor and metal mount give the lens a very proffessional feel.  Granted, you can tell the difference between this lens and the much more expensive L version, but as far as picture quality goes, the difference is not so obvious.Bokeh you obtain with such a wide aperature is terrific for portraits, at aperatures from 1.4-2.0 there is some softness to the lens which gives portraits a bit of a "dreamy" look.  This is desired from many, but I don't particularly care for it.  Nevertheless, shots become extremely sharp after 2.0 and continue to improve to f 8.0.As far as whether you should purchase this particular lens or the less expensive 1.8 model, it really comes down to personal preference.  Budget is an important issue as you can get one for less than $90 and the other for around 330. The less expensive one has a plastic mount, which wont last as long, and it just feels less solid.  It also has a comparitavely loud and slower autofocus motor than the 1.4, and the focus ring is nearly non-existant.It took me a while to choose between the two, mainly because of the cost,  but finally decided on this one, I have not been dissapointed. 