First off, I have to say that this is the first sims game I have ever played, so I can't compare it to previous sims titles. I also won't be going in depth into the features of this game because I figure there are plenty of other places to look for those. I can, however give an idea of what it's like playing the sims after having played more mainstream games.
My biggest worry about this game was that I would grow bored without a set progression. While it is definitely a play-style radically different from anything I've ever played before, it is none-the-less very entertaining. The goal is simply to make you sims (in game characters) successful in some way. This can mean making an army of friends or becoming an astronaut. Each sim gets one of these goals (called lifetime wishes in game) and has their lifespan to complete it. There are a lot of lifetime wishes to choose from and each offers different gameplay. This means there is great variety in how each sim gets played, and as a result, that the game itself is very re-playable.
That said, the open-ended, non-linear play-style won't appeal to everyone. It means the player has to take the initiative in everything. You could, if you wanted to, do nothing at all.
The other thing I was unsure about when getting this game was that sims have to complete a lot of very mundane task throughout the day. They must go to the bathroom, shower, cook, have fun, and receive some sort of social stimulation. As the player, you have to order them to do all of these things, and the monotony does get old after a while. You can set them to be somewhat autonomous (which means they will do something by themselves if left idle), but they are pretty stupid so you have to pay attention anyway. I will say though, that while this is somewhat mundane and repetitive, it does add somewhat to the gameplay that you have to juggle every aspect of your sim's lives to be successful.
Another thing I'll mention it what surprised me most. This game is not simply a simulation of real life, but more like a parody of real life. Sims make a lot of really comical mistakes, and they really are funny to watch. Also, the game features ghosts, teleportation devices, time machines (though you can't see where your sims go), and if you spring for the late night expansion, vampires. There are other things I won't go into, but suffice it to say that the sims world is very different from the real one.
Now, I did play only the base game without installing ambitions right away, just so I could see what it added. The base game itself is fun on it's own, but in my opinion, ambitions added a lot of stuff which should have shipped with the base game itself. Basically, it adds a whole bunch of career options for your sims. The nice thing is that most of these are a lot more involved than most other careers. A scientist for example will simply disappear inside the science facility all day, while the private eye (added with ambitions) has to go all over town collecting clues and solving mysteries.
The graphics in this game are pretty good, and will run on a wide range of computers. My computer isn't exactly a gaming machine, but it runs the sims 3 just fine (albeit on lower settings). The soundtrack is fun and upbeat, and you can play your own music with an in-game stereo if you want (using the custom music option). Another thing which is pretty neat is that there are no load times once you get into the game itself. Even though there is a pretty big town with lots of buildings to go into, you never have to sit and wait for a loading bar. On the other hand, you will be waiting for other things, like for your sims to wake up.
So, while I gave the overall rating five-stars, be warned that this is a very subjective opinion. The fun rating lost one star because of all the little mundane activities which I found kinda annoying. Overall though, I thought this an excellent game and would recommend that anyone interested try it. If you're really not sure, see if a friend has copy you can check out before buying your own. Also, my recommendation is that if you're going to buy the sims 3, you might as well buy the deluxe version right away because ambitions really adds a lot of good stuff.
Hope this helped.