I used Gleim, it has all the questions in the book and a CD. You can simulate the actual testing environment (I used Lasergrade)
It felt harder to me because it's broader in scope than instrument, but then again instrument is hard in its own way because it's more detailed in a way. I'd say if you're more of a person remembering general information rather than in depth info about certain area of study that it might be easier. Anyway, Gleim contains ALL of the questions, so...more time you spend going over the questions, better score you gonna get
I pretty much just went over all the questions and took a practice test 3 or 4 times and when my score was around 90 I felt I was ready...and my actual score ended up being 88. It's not too hard, just take some time to get over it and you'll be fine. Good luck.