I have been using Yoast plugin on several of my blogs and they seem to work good for me. I try to get my posts a little modified to follow up with their guidelines.
I like it and think it is worth it to follow their suggestions- they've made posts read/present better and I have seen tangible SEO results. I don't always optimize everything on every post but generally follow their suggestions. I use the free version and it works for me, but there is a premium version for $70 with extra features and support.
I've used it and it is helpful, although it seems to conflict with Jetpack in some ways - the social media auto-publishing tool in Jetpack seems to break when you use Yoast SEO. However if you're not publishing to social media, it is a very useful tool.
I've Yoast SEO plugin installed on all my Wordpress blogs, this plugin will cover almost my SEO onpage factors instantly and I don't need to remember each factor when writing the post. It's really worth to use.
With free version, you can optimize for only 1 keyword in the post, and with paid version you can do maximum 5 keywords in your post, that's a difference.
