Question Ranking
Pros
Pro Free and open source
Watir is free and open source, released under the BSD license.
Pro Easy to learn for Ruby developers
Since the test scripts are written in ruby, it is easy to learn, especially for ruby developers.
Pro Cross-browser
Watir webdriver is built on top of the WebDriver framework which can drive the most popular frameworks out there, making Watir perfectly usable with multiple browsers.
Pro Easy to create and maintain web tests
Since the ruby language is highly concise, the tests created using Waitr are very easy to create & update. Thus the long term maintenance of the test suits consumes less overhead.
Cons
Con Ruby only
Watir only uses Ruby as it's scripting language, so if you want to use another language other than Ruby or if you are not comfortable enough with it, it would be best to avoid Watir.
Con Harder to jump to any other library that exposes the Selenium API
Watir introduces its own API which differs from the underlying selenium-webdriver. That means jumping to any other library which exposes the selenium-webdriver API such as Appium becomes harder as paradigms can't be transferred between the two.