The game is a mixed bag. The Civics are a huge improvement over all other Civ titles. The mechanics behind religion are fairly positive, but creates easy ways to overpower other facets of the game. The way the game mechanics manage the size of your empire through costs are much better than corruption/waste. The Tech tree is mixed- without having the granularity of phases as Civ3 did, there is more diversity in what research choices you have, but at the same time there is more ability to min/max pathways, and also to research the upgrade for a military unit without having ever learned the original military unit. (cavalry can be had without ever having had Knights, for example). Terrain/map generation is poor at the moment. Mountains and Deserts are just wasted squares awaiting modders to fix. The "Worldbuilder" is a poor substitute for the terrain generator from Civ3. Again, modding tools are promised, but again, if modders are required in order to make the game deep, rich and balanced, that's a scathing indictment of the developers.
The place where this game really falls down is with the combat engine. Basing offensive power, defensive power AND health all in one number was really just an overly simplistic answer and creates an overly simplistic environment for warmaking. The unit promotions are great, and they even partially compensate for offense and defensive scores being muddled together, but nothing makes up for unit health and unit strength being coupled in one number. In short, the Civ3 combat engine was far superior to this combat engine.
In summary, in six months when the modders have had time to fix most of the problems, this game could be very worth playing. It's certainly not an unmitigated success at the moment, even after last night's patch.