I use the mobile app. It is great, but if you want to take full advantage of all the features (i.e. over analyze your chart) you need to log on from a computer. The only glitch with the mobile app that I noticed was when I was on vacation. I did not have phone reception but I continued to enter my data thinking it would sync once I had a connection, but it didn't and it deleted all the data I put in when I did not have reception.
I can also add, that I love the app. Not as pretty as some of the others, but given how great the web-site and service is, it is unbeatable.
I'm so glad I read this though! I'm going on holiday next week to South Africa, so I will be sure to chart the old way (pen and paper) and fill it in when I have service again. Thank you for the heads up!!