Works some of the time - but lets you down when it really matters.
The key one is the 'mirage' bus, one that is actually being reported purely from the timetable but appears to be from real time.
This manifestation can often be exposed by looking at the map associated with the bus - but, infuriatingly, not always.
The second, rarer but if anything more annoying, is the bus which is shown as continuing to its destination but in fact is being taken out of service as it arrives at your stop. Beware of Crawley Bus Station on that one.
Both of these happened to me recently : two 100s arriving at Crawley Bus Station in succession as per the real time info - only to be taken out of service, and at a nearby stop a mirage 20 bus followed by another 20 minutes later which blinked in and out of real time - but it in fact was real.
Apps have bugs now and again. What matters is how quickly the publisher fixes them. On that, Metrobus scores a one * only.