As a player of the Sims since the first iteration, I have to say it does keep getting better. Sure, there aren't a huge amount of improvements over Sims 2 (especially with all the expansions), but the level of detail is simply astounding.
Sims now "own" beds, preventing adult sims from sleeping in the kids' bed. There are many new ways to make money. The sims themselves are more intelligent in looking after themselves. There are many opportunities to see and loads to do.
The buy and build modes have been improved ten-fold. I personally like the option to move whole walls, objects included, without having to paint/move things around. This makes extensions to rooms incredibly easy.
The Create-A-Style is a godsend! Now you can change the way ANY object looks, and again, very easily.
Buy mode now has a family inventory, so if you move house, all your belongings are stored there, so there's no need to re-buy everything, and no chance of losing all your sim's masterpieces.
No loading screens and optimised gameplay also make this game easy to pick up and play, but be warned, there is no such thing as a "quick ten minute go" with this game.
For the people who report freezing and slow gameplay, maybe it's time to upgrade your PC, or at least experiment with the GFX settings. Windowed mode absolutely kills the frame rate, as does "smooth edges" (AA), so be sure to play full screen and don't be too proud to drop the settings a bit.
I'm running it on the below spec, and yeah, it's high end, but purely for comparison purposes, I run at 1680x1050, Full GFX, Full AA, Full Screen @ 90-120 FPS.
Intel C2D E6750 @ 3.2GHz
This game also runs on my work machine (E2180, 4GB, 9600GT,Vista x64) very nicely, although at a slightly lower resolution and lower GFX (no AA), I've yet to grab a frame rate though.
Overall, I can't recommend this game enough. Any sims fans out there will ultimately get sucked in. But if you haven't experienced the franchise before, this version is a great way to get into it.
If you're not sure about spending £30, then go for the Sims 2 bundle Play.com have going at the minute (only £7.99) and see if it's for you. I would just go for this version though, unless of course I had a machine more than 2 or 3 years old.