One good reason to use the IPhone (or other handheld PDA device) instead of/in addition to the Garmin is that it/they can use RASTER CHARTS...you know, those ones that look EXACTLY like the real charts (because they ARE).
Personally, I wouldn't use anything else but raster charts. And, yes, I know all about vector charts...too much, in fact!
Give me a chart which : (a) looks EXACTLY like the real paper chart; and (2) is, in fact, an exact replica of the official paper chart, not a knockoff digitized by some yahoo making minimum wage. That means RASTER. Can you spell it?
Sorry for the rant; just makes my blood boil to see some of what's going on in the new world of push-button yachting :-)