I don't know what $20 means to you. I don't know your tastes. I don't know how much free time you have. I don't know your history with the franchise.
This is how I feel. I'll just give you some key points to think about:
-Did you play Fallout 3 and New Vegas? Did you REALLY like NV? If the answer to both of these is yes, you're in for a massive disappointment.
-The game is bloated with filler quests. The infamous "Another settlement needs your help!" quests from Preston being the most egregious example.
-Your dialogue choices have no impact, ever. You can say just about anything to anyone and the result will be the same, barring a different tone of voice or slightly different reaction. A lot of the time you can attempt to refuse a quest and the quest giver will dismiss your answer and give it to you anyway.
-Quest design has little variety. The vast majority are just "Go here, shoot mutants/raider/ghouls, come back." I don't recall many quests having multiple outcomes.
The big take away from the game's critics is that you can't role play for shit in this game. That's Fallout 4's biggest crime, especially coming after New Vegas:
No quest in Fallout 4 offers nearly that amount of complexity. You solve most problems by shooting something in the face and looting its corpse. If you're ok with that, the game may be worth it to you. It offers a lot of content, well done combat mechanics, and if you're into it you can really get lost in the settlement building. Your enjoyment of Fallout 4 really comes down to what you expect out of Fallout game. If it's the ability to role play a character, it's a disaster.