System Shock 2 is totally playable still, especially with the HD texture mod - I recommend it to anyone interested. Played it for the first time ever last year.
The main difference between mods and the remaster seems to be that the enemies look strangely “smoother” lol
I really liked book 4, it felt closer to a return to the original Annihilation (though I also loved the more controversial 2 and 3).