I think it's a very solid game. Probably the best strategy game out for the XBox 360 (which I would say has a lot of great action games but a shortage of good strategy games). It provides a relatively easy introduction to the Civ series, and still feels very much like a Civ game, without a lot of the complexity of its PC bretheren.
I'd recommend it. I've played quite a bit since the UK release racking up 570 / 1000 achievement points. I'm a bit burned out on it for now, but it's definitely the type of game I can see coming back to on a regular basis.
For anyone who likes strategy games, but maybe has been a bit intimidated by PC Civ, I strongly recommend it. For existing Civ4 players, I still recommend it, but they need to realize what they're getting. It's basically a well polished, but simpler version of Civ4 which can be played in 3-4 hours from either your sofa with the TV or portable via DS. Some Civ4 players will probably have a lot of fun with it, whereas others will be a bit disappointed. Newcomers to the franchise will probably like it a lot.