Is it worth getting a function generator?
If you're working with digital systems and square waves/pulse trains only, then it's probably not necessary.  However, for analog amplifier design (ex. audio), it's an imperative.  

If you haven't been stymied on a project because you lacked this tool, you probably don't need it.  Spend your money on an oscilloscope and logic analyzer instead.  Conversely, if you can't imagine why you would need a logic analyzer, you should spend your money on a function generator.