Though you get an html file and a folder with all the external assets, which I find to be pretty annoying. Safari did a single .webarchive file, but I don't think anyone else ever standardized on something like that.
It often wrecks the layout, but I tend to print to PDF rather than deal with saved html files.
Save-As writing bunch of resources, that when loaded back into the browser might even be broken by lack of external resources are also, in my oppinion, a bad way to keep evidence (e.g. CYA). If you need it to persuade a non-computer-savvy person of something stated on a website, I'd say a bunch of random files in a directory is also much less convincing than a "printed-to" pdf, or screenshots you can easily mail around.
Regarding the .webarchive, I think IE supports another type of "web archive", with a .mht ending.
