My understanding is that it does have some value but the value of the link is very very very ......minimal. The value is diminished by the amount of other posts or links that are on the same page from other signatures. In my opinion I believe that there is more value not in the small tidbit of "link juice" that is provided by the signature link, but by the small chance of other users of this forum that visit the site out of pure curiosity or just by having an interest in one' services and clicking on the link.
Every backlink is a step forward on the road to success. And yes, it is better if the forum, where you're linking your website, is in the same niche.
In my experience, you will need lots of such links to improve your rankings. But I have also found that these forums will send quality traffic to your website.
Forum signatures have long been believed to have some value with regard to SEO and targetted anchor text. There's a great thread on SEO/SEM
From what I can see the value of Forum signature links, even if they are dofollow is miniscule. If you had hundreds or thousands the would add up to something. But every little helps.
what makes me not so fanatic in forum signature is that, forum has many outbound llinks. So, talking about link juice, that is not a great value after all
The fact that pretty much everyone in here has links to their site should answer your question. It definitely helps, especially on high PR sites that have been around a while.
If it is do follow also, we can't get any link juice from that particular forum. If our posts have certain quality in it and posted in relevant niche specific forums, visitors can click our signature and we can get niche-targeted visitors.