Yep, looking at the IL they are equivalent. And it definitely makes code prettier due to reduced parentheses. This is also true for generics.

Doing (x != null), !(x is null), or (x is object) all become:

    ldloc.s
    ldnull
    cgt.un
    stloc.s

why not add an operator so you can say (x !is null)?

Largely because (x is null) is a silly workaround to an uncommon and easily avoided problem (bad operator overloading).