So, I only needed to take the Reading, Math, Biology, and Chemistry portions of the this app exam. HESI in conjunction with the this app prep book by Elsevier really helped me a lot. I ended up getting a 94.5 cumulative score. 100 on the Biology, 92 on the Chemistry, 96 on the math, and 90 on the reading.
That being said do not expect to see the questions on HESI on the exam. The exam tests your critical thinking so, HESI combined with the book gives you all the basic knowledge you need but, you need to apply that knowledge on the test. It is also important to note that if you took bio and chem (or whatever courses match with the exam sections you are taking) and then you study with the prep book and app, you should be fine. But, be warned the chemistry section was very random and the prep was only slightly helpful. I highly recommend going over the periodic table properties thoroughly. I knew the biology so well because I just took Microbio prior to the test and had a huge biology review in the course. The prep and app are GREAT for the math and reading though.
All the being said, I studied for only 2 weeks and got these scores so, this is a great resource!