Ask HN: Modern Mental Arithmetic Methods?

I would like to improve my mental arithmetic. While okay in some areas, I am bad in others. I graduated in (pure) mathematics for my undergrad, while never having learnt my multiplication tables fully.

I have had the Trachtenberg book lying around for years, but I am not sure whether it is worth the time investment. I would also prefer having a computer programme which went through it, but I am open to doing it by hand if others can recommend it.

I would like to know what HN's experience is with improving mental arithmetic.

1. Have you always been naturally good? If so, what do you think about existing systems?

2. Were you bad but became good with a certain system?

3. Have you tried multiple systems? How do they compare?

4. Did you take classes or do it independently?

5. If system, what age did you learn?