I use 150, 210 and 360 convertible Symmars on 4x5 for colour negatives with good results. I have played with a borrowed 180 on my 5x7 conversion, but only looked at the ground glass - no pictures. The 180 has plenty of coverage for normal use on 5x7, but if you're fond of huge movements you might run out of image circle.
I can't say I've noticed more flare in the converted state, but I do use a compendium shade, and I have enough bellows to use the rear element on its own so the aperture blades help to screen the glass too.
In the converted state there's a fair bit of colour fringing near the edges of the image circle, and it doesn't go away with stopping down. How bad this looks will depend on the colour and contrast of what you are photographing, but it's more intrusive than a straightforward soft look. The single cells are pretty-well corrected for coma and astigmatism, so you won't get that sick-making swirly feeling to the out of focus parts of the image.