If you are getting signature links from relevant forum than it will definitely help in search engine ranking but if theme is differ than it will help in indexing the page thats it..
Like it's already been said. Every link counts. If your a heavy poster (13000 posts here...) then yes get those links in there. If you only post occasionally. Don't worry about it.
Every DoFollow link helps a little bit. Nobody could tell how much exactly. If the forum is related to your site it's a good way to drive targeted traffic too.
Its a double wammy, you receive a link for each post & also the potential for a visitor. Posting is great
It definitively helps and google counts this kind of links...I can see them in google's webmaster tools...if the page where the link is relative to your site in has PR, better
They bump up your keywords in the top of search engine result! Fore enous, the big G doesn't count no follow links.
Whether it's nofollow or dofollow the keywords in anchor text helps a lot in getting better rankings.
not sure how much it's worth. it's usually in my supplemental listings. but heck, i use it when i can.