I got a 2nd ps4 when they were highly marked down a couple christmases' ago.
My 1st ps4 has 5.05 and it is SO stable, I didn't want to mess with it. If you aren't satisfied with the update, simply put, you can't switch back.
The 2nd ps4 I was past 6.72 so I went to 7.55. It is less stable, it takes me 5 to 15 tries to load up the CFW, and only occassionally it shuts down. Not a problem, if you keep the external HD disconnected until you are completed.
If you are determined to upgrade (alot of games become available with 6.72 and again 7.55), try 6.72 first and see what you think. Then 7.55 if you don't like it, then 9
I also found almost all the games marked FW9.0 play in 7.55. Only COD Vanguard doesn't load, so far in all I've tried. I'm sure there is a few more, but I haven't come across them.