I use Navionics on a jailbroken 1G iPod Touch. Operation on an iPhone should be similar.
I am not a big fan of the Navionics package. It's ok wrt position on the chart, but the speedleader jumps all over, indicating wildly different courses. I don't think that this is related to the 3rd party GPS antenna that I use, but is possible in theory. The chart is small and requires a lot of zooming in and out - taking eyes off the water.
Regardless, I agree with SVAuspicious and rikhall in that bettery usage is heavy and that this app should not be used as a primary source of Nav. I use it when entering/exiting complicated or unknown harbors. My primary source of electronic Nav is a Mac running GPSNavX, which I consider an awesome value.... but that's a different thread.
Don't use an iPod or iPhone as primary Nav; you're better off with a paper chart and DR.
Sabre 38 "Victoria"