Picked up my copy of Sims 3 on the day of release and really haven't stopped playing it since. It is so much more alive than previous installments. Just being able to take your sim outside their house and explore the town with no loading screen is so freeing. The other big improvement from the Sims 2 is the fact that the townspeople (if you chose to keep this option on) can age along with you. Families move in and out, get promoted etc. Now people your sims know will age with them as basically your save is for the whole town not just the family you are playing.
For example after leaving a particular Sim family for a while, one of my sims had given birth to a baby boy and her mum was now an old lady. All this is optional and can be turned off if you like more control, but for me I think it will keep the game fresh and exciting so I am definitely going to keep this option on.
Some of the families i made in the Sims 2 weren't as fun to play as i thought they would be when I made them. If that happens in this game they will eventually age and die if I don't play them which is kind of like an automatic in-game clean up of boring sims.
Also the customization possible now is mind boggling, hair, clothes, furniture, even things down to the standard mail box you get in the game can be changed to any of the many colours and patterns. Makes for a much more interesting and varied experience.
There is still a lot to figure out and discover, I have only just scratched the surface but I have to give it a big thumbs up from my experience with it so far.