jQuery .with() to allow better chaining and flow
The code

I'm not quite sure of the purpose of this part:

if( temp instanceof jQuery ) {
    return temp;
}


This seems to be related to the idea of chaining methods, but it prevents me from doing something like this:

var width = $('ul').show().with(function() {
    return this.width();
}


Also, when chaining, there is no benefit of only returning the result if it is an instance of jQuery, because if you try to chain methods and you return nothing from with with(), it'll result in "undefined is not a function".

You also check if the fn passed to with() actually is a function. I wouldn't do that, since this will silently ignore a wrong parameter to with(), which is very hard to debug. Instead you can just disable the check, or even better do something like this:

if(jQuery.isFunction(fn)) {
    [...]
} else {
    throw "Parameter passed to `with()` was not a function: " + fn;
}


The idea in general

I have to side with @Esailija here. While there won't be a noticeable performance difference in any modern browser (if you omit the instanceof check), I can't see the benefit of replacing a local variable with an extra level of unnecessary indentation. But that is obviously a purely personal opinion.

Aside from the extra level of indentation, I find the code using with() harder to read, since it removes context from the individual lines. I would prefer something like

$targetEl.css("background-color", "red");


to 

this.css("background-color", "red");


any day, because in the former I don't have to scan the surrounding code to see what "this" currently means (which can be hard in JavaScript). Of course this is no concern in a snippet as short as your examples.