FIrstly, even if you COULD get offline Google Maps, it's not that useful in non western countries. Murmansk - the largest city in the arctic circle, is shown as just two streets in Google Maps. Most of their coverage of Central Asia is next to useless as well.
In addition, you need some serious storage for offline google maps. What I eventually settled with was NavDroyd - for Android, but there'll be equivalents for iPhone etc. It cost a few dollars, but not much - and the maps are all totally free, as they just download the free Open Street Maps maps, which are vector graphics anyway. It was fantastic. The interface was a little clunky to me and took a few minutes to get used to, but once I had it sorted it was brilliant - even in Siberia, Kyrgystan and Mongolia!
And being back in London it's pretty fantastic here too, although I find it easier to use Google Maps here as it inclues other information I look for (Latitude).