I have an iPad 4 and that is my primary navigation instrument, well besides the compass and charts. I will use the Navionics and iNavix apps. We are on year 3 of using the apps for navigation. We have an older, gray scale fixed chart plotter but we don't have the map chips for our local area. We only use that unit for the radar. We also carry and older Garmin 76CX handheld unit. You can download charts onto that unit but we don't have the disk any more so you can't update the charts. That unit sits in the ditch bag.
One of the keys is understanding the difference in the apps is the type of base chart you're using. There are raster image chart vs. vector image charts. NOAA has a good write up:
http://www.nauticalcharts.noaa.gov/mcd/learn_diffRNC_ENC.html
As far as the direct sunlight, the new units are far better than the older ones. I can see my screen fine in the direct sunlight if I lift my sunglasses (polarized sun glasses don't work with most screens). On my friends iPad Air it's even better.
We keep our iPad in a Lifeproof case that makes it water proof and pretty rugged. The only issue we have is that it will over heat if you leave it in direct sunlight.
We just use the internal GPS. But some of that decision depends on how you use your GPS/chart plotter. My primary use of the iPad is that it gives me charts that are very easy to zoom in and out. I don't rely on the location it gives me like that is the end-all-be-all. It's just one reference point out of many that we monitor while we are out. On longer trips and into unfamiliar areas we still use paper charts and will follow our course with visual references, compass bearings and the GPS coordinates to make sure all are matching up. Some of the power boaters I know will plot a course and keep their boat right on the line essentially heavily relying on the GPS only. Not my cup of tea.
We get about 8 hours of battery life. But we let it go to low power mode often. If you are the type that constantly look at screen, you will probably get less than 6 hours.
The biggest advantage to doing the iPad for navigation is the planning convenience. I can grab the iPad, Eldridge and cruising guide and made everything out. Using the boat information I put in it will tell me the estimated travel time, fuel usage and other useful info. The current info built right into the app is unbelievably useful for planning and you almost don't have to consult Eldridge but we still do most of the time. Really I could plan any trip we take with just the iPad using the Navionics and Active Captain App. There is even an animated wind grib built into the app.
Good luck and fair winds,
Jesse