When building back-links diversification is vital. Forum posting can be helpful with the condition that you must be willing to contribute to the forums that you plain to join. If you are just posting pointless questions or posts, there is a strong possibility you might end up being banned.

To answer your question forum is good and can be very helpful concerning acquiring links. But, I would not recommend you to use it as the only link building method. There are plenty of other good methods that you can use as well. Just use them in conjunction with forum posting in order to further diversify your link building strategy.