I am a fan. We use an iPad *without* a cell SIM, so just use the embedded GPS for when travelling and jump on wireless when in port. No cell or data plan and works fine.
We use the iPad with Navionics as a backup plotter and find it works very well. The Route planning addon (about $5?) makes for fast route planning - when trying to look at pros/cons of a number of different routes. We then create a final route separately on our main plotter.
GPS has been great. A few times out by 50' or so but not bad. We did find a few times up near Bella Coolla in the narrow fiords that it could not get a fix, but 99% of the time has been great.
As other poster have noted, make sure you get one with the GPS. The Apple store techs often don't know the details and get wireless location services confused with true GPS.
IMHO a low cost and high value addition. I think you'll like it.