I have a 28/1.8 that I use on my 5D (original). I like it quite a lot and use it more than any other lens. The 85/1.8 is a technically better lens, but the 28/1.8 is just fine and I find the 28mm focal length more useful for me. I recommend both lenses highly, and with your 50/1.4 you would have a great 3-prime set for full frame, for less than a 35L.
I don't find the corners of my 28/1.8 to be blurry at all - maybe I got an above average copy. The whole frame is slightly soft at f/1.8 but sharpens up nicely at f/2.2. I usually shoot it between 2.2 and 2.8. The lens really likes being indoors - really outstanding with available light indoor shots. Outdoors, harsh lighting can wash things out a bit. Just got a lens hood for it, which seems to be cutting down flare and helping the outdoor shots - recommend a hood definitely. Boo to Canon for not including lens hoods!