I took the TEAS last Saturday. I studied off the Mometrix Study Guide and I think it prepared me okay for the exam. I personally though the science section was the hardest. The study guide only covers a general overview, but there were very specific questions on the exam. For example, Mometrix doesn't cover the difference between osteoclast and osteoblast and yet I got a question about it on the exam. I recommend studying the endocrine system. I did get several questions on hormones, and what type of glands produced certain hormones. I also recommend studying the muscular system. You might get questions on what type of muscle produces a certain action. The question I got for example, was what type of muscle produced goosebumps during thermoregulation.
Mometrix is a good preparation for the reading, math, and english sections. The reading section contained paragraphs and asked questions pertaining to theme, main ideas, and opinions. I also highly suggest studying the different sections of a book, such as the glossary, appendix, table of contents. I got several questions on those. There are also questions asking you to read charts and tables. The Mometrix contains a pretty good review and explanations of different concepts you might see on the reading section.
Personally, the math section was the easiest section of all. The Mometrix was good preparation for this section. You are given basic multiplication, division, and various geometry and word problems. For example, you can be asked to calculate 0.3334 x 1.3232. We were allowed to use a four function calculator which helped immensely.
Even though I'm pretty good in grammar
(I scored a 750 on the SAT English section), the English section was surprising harder than I thought. They asked questions on commonly misspelled words, how to identify different parts of speech in a sentence, subject-pronoun agreement, how to identify grammatically incorrect vs. correct sentence, which groups of words don't belong in a sentence, root words, and so on. There are some questions that will trick you. For example, I got stuck on a question where they gave me different sentences and we were asked to order them into a logical paragraph. Do not underestimate the grammar section, study, and do lots of practice.