I bought the f/1.4 used from another member and while I'm not using it loads it is in my opinion enough of an improvement over the f/1.8 to warrant the extra cost.
It is faster to focus than the f/1.8, and more accurate (as others has said).
I've not notices any problems with sharpness wide open (although stopping it down it little does improve it). Bear in mind though at shorter focus distances (at f/1.4) the DOF is very slim which can give the general appearance of softness if the scene has lots of depth.
Build quality, while much better than the f/1.8, is still not startling for a none-L EF. I have the 85mm f/1.8 too which feels much better built.