I tried Yoast recently. At first, I really liked it. It creates a checklist at the bottom of your post editor that tells you if things are okay or if they need improvement. It'll tell you if your keyword's missing from the first paragraph or things like that.

However, after wrestling with it today, I'm not so sure about it because it's little checklist is dead wrong about half the items on the list:

# You have not used your focus keyword in any subheading (such as an H2) in your copy. > WRONG.

# Bad SEO scoreThe focus keyword doesn't appear in the first paragraph of the copy. >WRONG

# The images on this page do not have alt tags containing your focus keyword. > WRONG

Now I suppose it's okay if I know these things are fine but it's still a little disconcerting for me that it's so far off the mark.

I think perhaps people like it because it makes them double-check their post for things that affect their SEO but I'm tempted to just make a list of the things Yoast looks for and then do my own check manually instead.

Hope that helps.