The vast majority of readers can tell the difference between - and —. It's very noticeable. This sentence:

> I use dashes in place of commas, semicolons, colons, and brackets all the time - I just use the standard keyboard dash.

is noticeably weird-looking. Even a spaced en-dash looks a bit small; a spaced hyphen is just odd. But if you type it like this:

> I use dashes in place of commas, semicolons, colons, and brackets all the time—proper dashes, though, not hyphens.

it becomes much more conventional and legible. If you're going to use hyphens, it's better to admit the typewriter convention and use two, like so:

> I use dashes in place of commas, semicolons, colons, and brackets all the time -- I just use the standard keyboard hyphen, but twice.