On one hand, I really liked it. On the other hand, it's not really for kids.
Let's tackle the second issue first. Rango is certainly being marketed for kids - the trailer appeared in front of every kids movie for the past six months or so and Burger King is even giving away Rango toys (even though this is the least cuddly cast of characters since Deliverence). But unless your kid recognizes and appreciates references to Don Knotts, Lee Van Cleef, John Huston, Clint Eastwood, Hunter S. Thompson, or Apocalypse Now, he or she is probably not the target audience for this movie. And remember how confusing the plot of Chinatown was? Well, Rango is basically using the same premise. Throw in a bunch of killings and a pace that's more Leone than Pixar and you realize this isn't your typical animated fare (as an aside, what do we call these movies? They're not cartoons, they're not live action, they're animated but by computers... any suggestions would be appreciated).
Cautionary tales aside, there's a lot to like here. For starters, it looks amazing (and not a third dimension in sight, bless their hearts) and the soundtrack is really good (featuring one of my personal faves, Los Lobos). And it's kind of refreshing to see a movie like this that makes no attempt to cater to a younger audience (no Bieber references, for example). The cast, led by Johnny Depp, is top notch too.
So leave the little ones at home and take the older kids - just don't be surprised if they start showing an interest in Walter Brennan and rolling their own cigarettes.