7z is a must for de/compressing files. For years winrar was my choice. I am a perfectionist about the GUI and I could not stand the 7z interface and shell icons.
My use of file compressing/archiving software is simple, compressing jpgs and tiffs, archiving website projects etc. I can safely say now that the 7z barely wins on those tasks (a few kilo to 1 mega or more - this is based on usage simple observations and not on scientific comparison).
I wouldn 't switch from winrar for the GUI thing, but where the better 7z performance didn' t the "open source" convinced me. OK, I can live with the 7z file manager GUI, but what about the shell icons?