After using our ipads on-board both our boats for several years, and on charters in unknown waters, my opinion is that it is NOT a good plotter replacement.
There are some up-sides; the screen size/dollar ratio is awesome, and it has built-in connectivity, and the apps are decent. And its portable. And you can get a good waterproof case. But the downside are bigger.
1) Screen legability in sunlight. Sorry but this is a killer. Wait until you are in bright sun. Simply unreadable. Until there is a unit with a trans-reflective display, that not going to work.
2) Power. Full backlight, GPS on, and the app constantly re-drawing the screen sucks battery life. It always seems to die when you need it. And none of the cased allow you to charge and maintain waterproofness.
3) Mounting, I have the best RAM mounting cradle for the ipad, And I could never find a good way to mount it at the helm. Downstairs its great.
4) Where is the damn thing? Its value in portability is just as much a liability when it comes to keeping track of the thing when you need it. Kids?!?!?!?!?
5) Not great for Situational Awareness (SA)... yet. Downstairs for planning I like a north up, straight down view of the chart, but at the helm I prefer a good course-up, Gods-eye 3D view on new and tricky approaches. None of the existing app (AFAICT) to this.
6) General jack-of-all-trades, master-of-none syndrome. Its fiddly, the apps close when you don't expect. battery life, yada yada yada..... I like a device I ALWAYS know where it is, it ALWAYS has power, and ALWAYS is working doing its only real job.
SO my net-net, great as a companion app, most notably if your plotter supports a mirror or data share with a branded app, most do now. But for your sole plotter? Not so much.