I went into Rango pretty cold, and there's a point in the movie where I had to ask myself:
I've read a few reviews that claim Rango isn't for kids, but there were plenty of kids at the showing I attended, and they had no trouble following the story (and explaining it to their parents) and weren't bored by it. The reviewers were probably just confused because there's plenty of material to amuse adults as well.
There's also nothing in there to horrify the under six set (unlike, say, Toy Story 3).
For the gamer crowd, it wouldn't take much work to convert this into a Zeppelin Age or Space 1889 adventure, so you can justify seeing it for that reason if no other.