If it's free, do it. The more prep you do, the better.
What I think you should look for in particular is a site that tells you what TYPE of problems will be on the test. Then, make sure you know how to do each type of problem. For example, make sure you know comma rules. Make sure you know how to solve linear equations (or whatever math they have on there; I totally forget). That kind of prep, I think, was the most helpful for me when I took the test.
It's not like a test where you can study the material that's going to be on the test. You don't know what will be on there. But you do know what grammar rules, math problems, and what not they'll expect you to be able to solve, so make sure you know how to solve them.
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Some things are meant together, some things are better apart
Some things are easy, when other times they are hard
But that doesn’t mean what’s hard isn’t what’s meant to be
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Al Lewis