They do not work well enough to help posters learn of previous posts.
Title, Topic, Author, and Source searches are unreliable.
To searches are fairly reliable, while By searches are not.
If a Freeper wishes to post an article, there should be fields of an initial page design, wherein the Title, Source, and Author may be entered, then searched upon, with results listed starting with exact matches running on down to "sounds like" and / or [is] "spelled like."
Again, there needs to be a "Posters' Search for Previous Posts Page" at FreeRepublic.com.
I also notice that all the folks who scream "already posted" fail to notice this.
Yes, they do -- IF people use the correct article title!