Seeing all the negative, discouraging reviews for this game, I decided to write my own, and give my two cents.
I've played the original The Sims and a couple of the console spinoffs on and off for nine years, give or take; I guess I consider myself a fan of the series. I held off on buying The Sims 3 for the longest time because, like many other people, I wasn't willing to spend the absurd price for all the "expansion packs."
As a game, The Sims 3 is far superior to the original. The AI pathfinding is fantastic by comparison- Sims won't get stuck in doorways for hours on end. The neighborhood is large and open enough to feel like a real, living, breathing world, complete with shops, sales, parks, and tons of activities- a huge forward step from the neighborhood of the original. Instead of having to call up a friend or a community shuttle, you can walk or drive virtually anywhere in the neighborhood. It's stuff like this that makes your Sims' lives feel more real than in the first game.
There is a massive amount of customization in this game- from clothes, to cars, to floors and walls, virtually everything can be personalized with custom color schemes or patterns.
Instead of the "Personality Points" of the earlier games, The Sims 3 allows you to pick traits for your sims that make them more interesting or unique, and allows for more "personality" than earlier games.
There are, of course, plenty of problems to be had with the game. The "expansion packs" are ridiculously priced (I wouldn't even bother, but then again I can't really say) and there is no "complete collection." To buy all the expansions and stuff packs would easily set you back a couple hundred dollars, which is absolutely ridiculous. There's plenty of game in this game without the expansions, though, and the modding community has made lots of neat stuff, all free of charge.
The game relies more on "Moodlets" than your sim's actual condition to determine mood. This is another complaint I have with the game- keeping mood at the maximum is extremely easy. A good meal and 8 hours of sleep is enough to keep them at full mood really no matter what happens throughout the day. The Sims has never been a "hardcore" game, but there isn't really any challenge at all in this game, aside from maybe some initial budget issues and relationships. While this does solve some of the issues of the original game, like constantly having to juggle things like Hunger and Energy to maintain a good mood and have any free time, I feel like it should have been implemented in a more subtle way that would require a bit more attention to be paid to your sims' needs.
I bought The Sims 3 when it was on sale, and I can safely say I wouldn't regret paying $19.99 for the base game if I had to. If you're a fan of the series, I'd say grab it. Unless you have a couple hundred bucks to throw away, though, steer clear of the unnecessary expansions and microtransactions. In the end, Maxis made a quality game. It's just a shame that EA's dubious practices have gotten to be what they are.