First of all, still I am a fan of YUI libraries. It is an open source client-side library and provides rich set of UI components (like Datatable, Uploader..etc.,) and it was actively maintained by YUI team. but now YUI announced and decided to stop all new developments and will do bug fixes only if anything critical to YUI properties. So I will not choose/recommend YUI for coming projects.
Below is a important points from YUI announcement.
Therefore, we have made the difficult decision to immediately stop all new development on YUI in order to focus our efforts on this new technology landscape. This means that, going forward, new YUI releases will likely be few and far between, and will only contain targeted fixes that are absolutely critical to Yahoo properties.