I have been using Navionics charts on the iPhone since last summer- East Coast, West Coast, Mediterranean, and now Caribbean for charter next month. I'm impressed. That said, the brand new Practical Sailor review recommends caution regarding the iPhone's use for serious live navigation. The GPS receiver is apparently not multi-channel but instead rapidly samples several channels. My only concern is the tiny screen size. So far it has been spot on when I have used it. My wife used hers last fall when crewing Seattle to San Francisco because the skipper didn't have his chartplotter configured for what she wanted. Worked great for her.

We'll put ours to the test in the BVIs soon since the charter boat probably doesn't have a chartplotter on deck. Of course we'll have our paper charts on deck as well. Since the Practical Sailor review mentions that we iPhone users are cult members, I guess we'll have to sacrifice a goat while using ours {grin}.