It's a great tool to not only learn N1QL but also to try out and optimise queries before using them in your application.
Depending on your operating system, and how you've installed Couchbase, the location of cbq can vary. Here's a quick reference for finding cbq on your machine.
Mac OS X
GUI tool: Query Workbench
If you find cbq useful for experimenting with N1QL, Query Workbench is where you're more likely to spend your time building and refining your queries. It provides a GUI environment with several coveniences, such as query autocompletion, an overview of the index status of your buckets and your results on-screen at the same time as the query itself. It's in developer preview now for Mac, Linux and Windows.