App Insights is awesome for integrating into all sorts of Office 365 and SharePoint dev things. By simply dropping a couple of statements in your code, you get to report on it later and get way better visibility of what is happening than you might otherwise have. Whether it’s executions of your web part, errors which are happening, or how often users are going down particular paths in your app, it’s a great way to build this logging in with low effort. In terms of practicalities, you do need an Azure subscription of course and App Insights is chargeable if you pass more than 1GB of data per month (at the time of writing), and your data is only retained for 90 days. BUT, you don’t need to build any kind of front-end or query tool, you can get graphs/charts, weekly summary e-mails, and importantly, alerts if any conditions you define are not met. Additionally, there are lots of ways to integrate with the data (including BLOB download to keep the data for longer). All considered, the features are pretty awesome.