I used one frequently when my work was mostly repairing the company's main product. You already know that the design works, and after a while you get to learn what each signal should sound like (if you get a probe, get one with a beeper). It saves a lot of time to be able to probe around without having to look up at a screen.
For new designs, they aren't very useful. Knowing that there's some sort of signal there doesn't mean it'll be the right signal. I haven't used mine since I stopped doing repairs. Even the cheapest oscilloscope will be much more useful.