cooper250: I took it. Not that easy. Some tricky multiple-choice questions. Definitely study the manual, and do the workbook if you have it. Do the sample test if you have it. It is worth it to buy the sample test they have on-line for sale, so you can get used to the way they ask questions. Don't memorize the more complex formulas, but do know the basic ones (e.g., how to calculate ideal bodyweight). Similarly, you don't have to know all the muscles, but do know the biggest ones (e.g., the names of the 4 quadriceps, the rotator cuff ones, etc.). They ask questions about all areas, so pay attention to ethics, assessment, etc etc.
As for "easiest". there are lots of easier ones. They're just not accredited and recognized like ACE. I think ACE used to be easier, but they revamped their program. I think the pass rate for the exam is like 66%, so it is no cake-walk. I don't think there are many gyms that don't recognize ACE nowadays. Most places will just be glad you have ANY reputable certification.