I got into the final wave of the beta over the weekend.
Absolutely buying this on release day - gorgeous game and IMO a large improvement over previous installments. The only things that bother me is the limited size of the map (unsure if it's a beta thing or not) - if I can't build a megacity I'm going to be sorely disappointed.
Traffic model, economic model, new zoning model, and new utilities model are all IMO simplifications that make the game more fun, but does not remove depth. For those who haven't followed the game: power, sewage, and water no longer require power lines/pipes/etc, and travel along roads. They propagate along roads outwards from their generation source (water towers, power plants, sewage plants, etc) until their capacity is reached. There's a great animated overlay view that lets you monitor the state of each of these pretty easily.
The biggest pleasant surprise was how well it ran. I pushed all settings up to max and it didn't even flinch. SimCity 4 in contrast got insanely slow when your city became even slightly large - even on today's hardware.
The ability to monitor cashflow is also a welcome change. Instead of having to dive into your balance sheet to figure out your fiscal position it now shows a very clear flow rate (negative or positive) next to your balance.