WebmasterQuery

Yes, signature links are awesome for Link Juice! Especially on older forums with a lot of PR (high PR).

Try to find dofollow forums as well. This will give you even more link juice.

Another thing that forum signature links do is actually drive traffic to your site. If you make interesting posts/threads, people will click on your signature links.

I think they are. I was warned not to use the same keywords at every forum site I visit. However, I think they would be very effective if you wanted to gain more backlinks to your site, for instance.