I've been using one for years and encourage my students to. The minute the student finishes training they will never look at a chart again and will be following a magenta line on SkyDemon or something similar. NATS and the CAA have been quite clear - almost all airspace infringements are committed by pilots not using a moving map. Yes, you need a plan "B" in case of failure - but that is usually "get the phone out".
I've had iPads crap out when they have been left on the glare shield on a sunny day while we go for coffee and cake - so I don't leave them in the sun any more! I've also had them fail on cold days in open-cockpit aircraft - that can be mitigated by keeping them on charge.
Full size ipads tend to be a bit big - I use an ipad mini or an 8-inch Android tablet.