Yahoo answers and answers.com and some other answers sites give back links but mostly nofollow.
Mostly you got nofollow back link from answers site, these are give good result in traffic point of view.
mostly back links from question answer sites are nofollow but still it is useful in traffic point of view.
Its not an answer site, but webmarketingblips seems to work well for me.
And if you are looking to optimize your links, here is a good guide: SEO Link Optimization
so if it`s no follow the only value of them is to get a little targeted traffic
Yes, Backlinks helps dramatically, check out my blog posting about backlinks, I also have some other articles available about starting your own blog: 5 Great Tips to Getting Back-links at :http://tech4000.blogspot.com/2013/01/5-great-tips-to-getting-back-links.html