IMO, the best Fallout game made, and one of the best rpg games in general ever. Gameplay was slightly improved upon from original, there were new perks/abilities, and most importantly, a story that challenges (if not plummets to death) all the other Fallout games to date. The story is fun and well written - not to mention very addicting as well.
Now, why do I say it's the best Fallout game?
1. There are many quests, in fact the only way to complete them all is to beat the game multiple times. There are also multiple endings as a result. But it never makes the game boring.
2. Writing is on par with the classic fallout games (Fallout 1 and 2). None of the mediocre Bethesda writing that can be seen in 3, 4, and New Vegas is to be seen (mainly since most of the NV team is from Obsidian, the company with some of the original team that made 1 and 2). Characters are all interesting for the most part as well.
3. Like I said, gameplay is slightly improved upon, which makes the game more fun and satisfying to play through. It's not groundbreaking by any means or much different from 3, but its good.
4. Total Freedom. You can basically do what you want in this game. And not just the different factions/quests you can choose. I mean basically everything. You can kill anyone, you can choose whatever quests you want almost, etc. But your actions will have consequences, good or bad, that may help or prevent you further to beat the game.
5. DLC is really good as well. Best collection of DLC in any fallout game.
Now, for the cons. And yes, I love the game, but there are some problems that can be annoying.
1. Bugs/Glitches/Crashes: Make sure to quicksave a lot, in case some of these appear. I haven't had any of these that broke the game, but they need to be fixed. There might be some game breaking glitches/bugs, but it's still a low chance to get something that bad. Crashes while traveling the wasteland can especially be annoying. Not as bad as 3, but still not really acceptable.
2. Graphics: I'm not a person that criticizes a game on it's graphics, but they are basically the same as 3. They haven't aged too well, but we're decent for the time.
3: Difficulty. Since the game gives you a lot of freedom, you may not be prepared for the middle to later game. Even when on a decent level, it can be very challenging (ex. Sneaking past Deathclaws that can kill you in one hit or almost, escaping B.O.S base without a stealth boy when they're all hostile). Learn from your mistakes, and quicksave constantly. Thankfully, they are not impossible and may not even be a challenge when your at a high level and have OP companions.
*Difficulty being hard can be a subjective "con".
What are you waiting for, buy it! You won't regret it! The Pros significantly outweigh the cons.