When would I want to use Slick and when would I want to use Sizzle? What are the pros and cons of each as a standalone CSS selector engine? How easily can Slick be used with JQuery? Can Sizzle be used with Mootools?
http://davidwalsh.name/mootools-sizzle - using mootools with sizzle.
valerio posted about why mootools in particular was never going to use a 3-rd party project like sizzle - lookie here http://ajaxian.com/archives/mootools-and-sizzle
since that post, the need to scale the mootools selector engine must have resulted in the advent of slick (which is a standalone engine but one that the mootools team develop so its easily maintainable / patchable)
you can easily checkout slick and port it into whatever framework as well. here's a gist on using slick in mootools 1.2.x - http://gist.github.com/361474
interesting posts on use are around, http://davidwalsh.name/elements-shorthand-slick for example but it's all early adopters stuff so before 1.3 is out proper, i don't expect it getting too much attention.
as for performance, its nearly identical from some beta tests i saw a while back with sizzle getting ahead slightly on things like document.body. anyway, post your findings, would be interesting.