How effective is guest posting ?
I tend to think that the effectiveness of guest posting is about what you put into it. If you guest post with the mindset to get it down as quickly as possible, make it thin / short for the sake of getting the benefit of the backlink, then don't expect to get much out of it. More than likely you'll get penalized for not following blogging best practices.
So with that said, the quick answer is that guest posting can be extremely effective, but you have to do your due diligence to write blog worthy content because remember... guest posting is still a blog post that you're posting on someone else's website for their readers to read. If they like your content, they are that much more likely to head back over to your blog to read other content that you've written. Making a good first impression with your guest posting can lead to more traffic opportunities and referral traffic. A poor first impression won't be helpful to you at all and likely won't lead to you being offered other opportunities to guest post for others in the future either.