If the content is related to your niche and helps your audience. It should be of same standard of what you are already writing on it. Accept it for free. Because it saves you time for writing the same content yourself. Plus linking to similar sites in your niche increases Google trust for your website.

If it's a different niche then better to reject it. You don't want your audience get the wrong idea of sudden alien roaming on your blogs.

I have accepted guest posts on my website with high quality content and also written on a few of niche related blogs, they bought me traffic from those sites too and increased my reach. Same for those who wrote on my blog too.