It's effective. But it also depends on how big the forum is and if it's relevant to the topic of your board. Try to be clever and find ways to plug your site on huge forums. ^_~
All types of backlinks count for your SERP.... period... some more though, some less... forum signatures is on the lesser side of course...
If the forum allows dofollow links in signatures then it will help build backlinks and if its of similar niche then it should get you decent amount of traffic as well depending on how active the forum is.
Yes they do count as backlinks......and will help you a lot but first LET THEM GET SOME AGE and with some age they will get some authority and PR and will help u
it helps for SERPs, i and my competitors have proved it but there are many better ways to get quality links, but dofollow forum sigs help.