I took REG on 8/21 using GLEIM. It was my last test and GLEIM was significantly harder in REG than in the other 3 sections. I felt nervous going in, but walked out confidently. GLEIM adequately prepared me for all4 tests and both CMA tests. Passed all tests I've gotten scores for first time. Just hammer through MCQs and don't be disheartened. GLEIM is significantly more difficult than the exam is.
To clarify how I prepared for REG, I read the book all the way through. Then I did about 1250 MCQs beginning 8 days before the exam. Did like 4 Research SIMs to familiarize myself. Other than that, I did 0 SIMs. I felt GLEIM MCQs are adequate preparation for SIMs in all 4 tests and specifically practicing SIMs isn't necessary.
I used Gleim’s test bank for REG (Roger was my primary review course), and yes it’s definitely harder than the exam. Also don’t be discouraged by many of MCQs in the test bank, as many of the questions go into much more detail than the exam ever will. I also practiced a couple of research SIMs for the feel, and I think 1 or 2 SIMs at random. I agree with AF , SIM practice isn’t necessary.