I have been using an iPad as a backup plotter for years on my trawler. We always carry it on the Parker, just in case.
The trawler has a Raymarine C-120 plotter and the Parker has a Lowrance. Both use Navionics. The iPad has the Garmin Blue Chart app as well as the Navionics app but the Garmin is our go-to app. Just personal preference.
I will say that the Garmin is far superior in the Bahamas. We turn off the plotter and run off the iPad when we take the trawler over there. The Navionics shows acres of nothing while the Garmin has good detail. They use the Explorer Charts data.
A good friend just put Raymarine on his trawler and he loves the fact that he can do everything from his iPad wirelessly. And while I wouldn't bet my life on it, the Auto-route feature is pretty neat. Even in Penobscot Bay, with all its rocks, ledges and tiny islands, it does a grea job.