From what I see the maps stored on memory are not worth it and doesn't include navigations. Since good navigator requires online,something I can't get outside my country, it's pretty useless.
I'm gonna give up on BB, bought it especially for maps just to find out it's one of its weakness.
U may pan (at your preferred zoom level) the destination area you are going to visit and BB will save it in its cache/memory. GPS will work on the cached/saved maps even when there is no network. You will see were you are or going but you wont get any directions calculated. I once used this when I had messed up my APN settings but didn't like it but had no other choices.