Using Google maps 5.0 offline works great!
About a month ago, Google rolled out a big update to maps (5) that included the ability to cache. I'm loving it. It's super fast now also - according to Google, they estimate that it uses 70% less data than before because of the way the map graphics are generated (they're vector based now). For people who travel a lot, like myself, this is a lifesaver and means I don't need to worry so much about having roaming data. I'm very happily amazed, and I feel sorry for all those stuck in the past with their iPhones
The 3d building rendering is pretty swell also, but right now in the UK it's only available in Birmingham and Cardiff.