Io is a very interesting language for people interested in languages, 
especially of the Object Oriented (à la Smalltalk) kind. That being 
said, if you intend to use it as a practical language, it has two major 
problems:

1) performance is not a goal of the reference implementation, or at 
least less than simplicity and ease of comprehension;

2) although there are a few libraries here and there, the ecosystem is 
not on par with other scripting languages.

All in all, in the field where it could shine - as an embedded scripting 
language - it ends up being "inferior" to its main competitor - Lua - in 
almost every dimension.

That being said, I think Io is a beautiful piece of software which you 
should definitely study if you are interested in languages design.