My coworkers and I have a fair amount of experience now using iPads as GPS tools. We use LifeProof cases and large external batteries of various types to recharge them on the go. They are great for specific tasks, but are not in any way a replacement for a traditional GPS in our experience.
Positives:
Large moving map screen is ideal for "big picture" navigating. Particularly at speed (we use the from helicopter seats quite a bit).
Can cache sat imagery, which is great for "looking" over the next hill or treeline.
Some of the GIS/GPS programs allow for great data gathering, including taking extensive notes and photos attached to a waypoint.
Allows easy route testing. A few swipes a finger will lay out a potential route and show you the distance, elevation gain, etc.
Negatives:
Even with the latest OS that allows use of the GPS in airplane mode, they eat battery really fast when being used continuously. Takes a lot of juice to run that big screen. Phones have the same issue if used continuously.
No way to stick them in a pocket, so you are either holding it in your hand or putting it in your pack. Major pain when brush crashing.
Heavy. Particularly when you figure in the external battery pack large enough for a few tablet recharges.
Long story short, tablets are great work tools when gathering data is a first priority, but I would never consider one for hunting or backpacking. Just way too un-handy in the field.