I own the 1.4 - can't comment compared to the 1.8 as I've never owned that lens, but I do love the look of the images I get from the 1.4
I bought mine specifically because I had a mate's wedding to do - and it was recommended by a bunch of wedding photogs as a 'must have' lens - at least that's what I told my wife! LOL
I used it on the day, and it gave me the dreamy narrow DOF shots I was after, and allowed me to work without flash at the reception, in very low light - definitely a winner.
Since then it's done great service - mixed of people shots and more general stuff - even did a day out shooting landscape with it as a change of pace for myself.
Construction is ok - not an L lens, but it's a lot cheaper. AF is a little noisy, but seems to be fast enough - I use it a lot on AI Servo when trying to keep up with kids!!
When you're at f1.4 the DOF is almost nothing on a FF camera, slightly more on a crop sensor - although still wafer thin, so extreme care is needed to get the right bit in focus.
BOKEH is lovely - which is the whole point I think.
Any way, picture worth a thousand words etc etc - some samples:
From the original wedding day shoot -
And other stuff: