When should i use Celluloid::IO?

Unlike systems like Node.js, Celluloid does not require that all I/O be "evented". Celluloid fully supports any libraries that support blocking I/O and for the overwhelming majority of use cases blocking I/O is more than sufficient. Using blocking I/O means that any Ruby library you want will Just Work without resorting to any kind of theatrics.

Celluloid::IO exists for a few reasons:

• During a blocking I/O operation, Celluloid actors cannot respond to incoming messages to their mailboxes. They will process messages as soon as the method containing a blocking I/O operation completes, however until this happens the entire actor is blocked. If you would like to multiplex both message processing and I/O operations, you will want to use Celluloid::IO. This is especially important for indefinite blocking operations, such as listening for incoming TCP connections.
• Celluloid uses a native thread per actor. While native threads aren't particularly expensive in Ruby (~20kB of RAM), you can use less RAM using Celluloid::IO. You might consider using Celluloid::IO over an actor-per-connection if you are dealing with 10,000 connections or more.
• The goal of Celluloid::IO is to fully integrate it into the Celluloid ecosystem, including DCell. DCell will hopefully eventually support serializable I/O handles that you can seamlessly transfer between nodes.

All that said, if you are just starting out with Celluloid, you probably want to start off using blocking I/O until you understand the fundamentals of Celluloid and have encountered one of the above reasons for switching over to Celluloid::IO.