I've not intensely tested the X5S recording formats, but my previous experience with the X5 at 1080p did lead me to test 1080p @120fps and I found it to be pretty terrible as before. Footage is buttery smooth with the high frame rate, but image quality seriously suffers. Looks very soft and almost blurry without being focus related.
Incredibly dissapointed. Image quality of 4K video is much, much better and unfortunately I don't think the frame rate makes up for the significant drop in image quality for 120fps to be all that usefull in proffesional production.
Individual taste may vary - but I'll be steering clear of anything other than 4K25-60.