If you're going to use UNIX for anything other than just email or if you ever expect to use other UNIX systems, yes. If you're only using UNIX for email or hope that you never have anything more to do with it, no.
Most people find vi somewhat hard to learn.
This is the default when you enter vi. In command mode, most letters, or short sequences of letters, that you type will be interpreted as commands, without explicitly pressing Enter . If you press Esc when you're in command mode, your terminal will beep at you. This is a very good way to tell when you're in command mode. In insert mode, whatever you type is inserted in the file at the cursor position.