I think it's great...I enjoy it more than TS3. Arguably would say that gameplay in TS2 is better, but the build/buy and CAS stuff of TS4 is leaps and bounds above TS2, and the Sims themselves, with multi-tasking and all of the facial expressions and animations of TS4 are better in gameplay.
Having played TS4 for a while now, I finally did remove both TS2 and TS3 from my PC. I just can't go back to them anymore because I miss the faster load times, multi-tasking, and better build/buy and CAS items of TS4. But yah...still do miss some things about TS2 as well, just not enough to play it instead. Multi-tasking is really a game-changer and the toddlers in TS4 now that we have them are soooo adorable.
I will say, however, that before a bunch of the EP's and GP's, I still played TS2 and TS3 pretty regularly. So TS4 is definitely a game that really blossomed as more was added to it.