Played this pretty much all weekend (actually didn't play any WoW... WOW!) and I'm really, really enjoying Sim City 4 a lot more now. Without Rush Hour I tended to get bored and quit within a short time period. With the expansion I'm playing for several hours straight and then thinking about other ways to do things, etc so I go play some more.
The U-Drive-It missions are a fun diversion, certainly not the meat of the expansion IMO. The best part is having the 3 difficultly options and all the Route Query tool (AWESOME, so so needed!). I find myself using the Route Query tool endlessly.