Having your own sites in your sig and making helpful posts is a way to build a brand, but if google doesn't devalue the hell out of forum sigs soon I'd be shocked. As alluded to above, blog commenting was abused, and blog commenting was massively devalued.
People that join forums solely for link value are useless, not to mention obvious. People that sell services "link building" in forums are worse than useless, they're a cancer that should be cut out as soon as identified. They add zero value and sometimes even do their client a huge disservice by making mention of his URL a bannable offense. Ask any moderator what they think about guys that have a sig pushing "Ugg Boots". if they aren't spamming with that ID, they're doing it with another.
I banned a moron from PK recently who had a sig pushing a Houston business. All his posts were political, and the last thing a guy with a brick and mortar biz in Houston TX really needs is a few hundred links identifying him as hating Americans and idolizing Saddam and bin laden. Not helpful should his clients see that. Whatever idiot he had running his website contracted some we-be-cheap link building service and fully deserved to be fired for it.
Using forums for SEO value only does for forums what puppies do for carpets. I for one ban guys like that without hesitation.