As many have stated before me, the game is an improved Fallout 1 and really well done so. It's bigger, feels deeper and the immersion is even greater! Items are expanded as are locations and NPCs. The plot is also engaging! The AI is improved but you'll encounter the occasional "funny" moments (like enemies staying near your companions and your companions just staring at them instead of attacking, or companions using a spray attack on each other etc) but those won't ruin your experience even the slightest. Also now you can customize your companions better, since you can give them weapons and armors to wear, plus they can improve their abilities the longer they are with you. Get it!!!
NOTE: Some people have Fallout 1, 2 or Tactics in their library but still seeing those as "purchaseable" in their respective pages. The reason is most probably because they have the "classic" versions in their libraries and not those "new" ones. For more info check this link: https://www.gog.com/forum/general/rerelease_fallout_series_72e26/post121