Google Maps on the iPhone is great, if you have data connectivity. If you don't, it's worthless, as it does not cache map data. Make sure to have an app that caches map data and cache the data for your AO.
Got any suggestions?
Anyway, if data is down, I think that means GPS for navigation on any map is down. For map only without GPS, I'd prefer to use a paper map and conserve my battery, but I guess it wouldn't hurt to have on my phone a pure map of my then current area.