It's a great tool for bloggers who wish to work out with SEO, but more important, it's a way to get new readers. At first, I was interested in SEO stuff only when it came to guest posting, but later on I realized that by providing true value to readers of other blogs, I show them that I'm a serious deal, and after first professional and serious guest articles, I've noticed a great increase in traffic and and the number of active readers on my own website. I recommend it to everyone - both beginners and advanced bloggers!
I guest post at least twice a month, but if you can post a guest article once a week on a new blog, then it's definitely worth it. And I accept guest posts for my own blog, because it's beneficial for me, too - if others write for my blog, I can focus on writing for others :).
Usually, I use Google to find new blogs, but I also scan the comments on my website to see if there are some fresh bloggers I can post with, and also I use my Facebook page to ask for guest post opportunities :).