It is a neat app that I find interesting and entertaining. But it's important to keep in mind that it is not an actual (or necessarily reliable) substitute for an AIS receiver. The reason is that these apps are far too dependent on intermediaries. They are an extremely roundabout way to get AIS signals. AIS is a very valuable navigational aid that enhances safety on the water and in my opinion, an AIS receiver is the only way to go, particularly as they are relatively inexpensive.
These apps depend on someone with a shore based AIS receiver to collect the ships' signals, which are then fed via computer Internet connection to the server that feeds the app. There are tons of these shore based stations around the world that feed the app. The server is likely on another continent.
So the quality of the AIS information you get through these apps depends on:
1) the quality of your cell signal, if you can get one;
2) the location and quality of the equipment (antenna, receiver, computer etc.) of the shore based station and whether it is powered on at any given time;
3) the quality of the app and the server, wherever it may be in the world.
This is roundabout and unreliable because ships send their AIS data via VHF radio signals. Because commercial ships are so big, their antennae are very high and therefore their signals are easily picked up from 10 to even 20 miles away - plenty of time to allow you to get out the way if necessary. My AIS receiver was about $125 and my 18 inch gunwale mounted antenna was on sale for $5. Best of all, the receiver easily connected to my chartplotter and not only does it show the AIS transmitting traffic, and all the associated information (name, speed, heading, destination, MMSI etc.), I can set proximity alarms so that I am alerted if any ship moves into a collision course with me.
An AIS receiver ensures that the information you get is current and reliable. It's not radar, but it's about 10% of the cost.
The AIS apps are interesting and fun, but I don't think they are a substitute for a receiver when on the water.