I think guest blogging/bloggers are great.
Its kind of nice to get a refreshing take on things for your blog.
I haven't personally guest blogged yet, but would love to:P
Love guest blogging and having guest bloggers on my blog. And yes I would write about the topic of my own blog so it's a relevant link back. Guest blog posts should be unique and never posted anywhere before.
One of the guest blog posts I wrote was just published today if you're interested in reading it on the Reggae Marathon blog.
Personally I look for guest bloggers who have a blog in the same niche. I would not accept a weight loss/fitness post from someone with a blog about dolls or other various topics for example LOL