Using Rails as a go to thing for creating API's or full blown web apps.
Love writing Ruby, convention over configuration style is great and it has a tons of gems and community behind it :)
When ever I need heavy user client side apps this is my tool of choice. There are a ton of JS frameworks out there, picked this one because of philosophy they are trying to put out there and great community. Two way data binding FTW!
Easy deployment for kicking off your project. No DevOps headaches.
Good for deploying apps where you want total control over everything. You need to have DevOps and SysAdmin skills tough.
Easy to spin up UI for anything. Great for testing and mocking stuff out.
When ever there is a need for real time, event driven , non-blocking things this is my go to. It filles in the spots where Rails could not and it does that very well.