This is a cheap and easy way to get preacher like isolation on your biceps. You really can feel it when you're towards the end of a set, when that final rep takes 10 seconds going up because your muscles are so fatigued. I have never been able to get there without a preacher bench until now. This results in really strong DOMs and you know you've really worked the muscle.
Assuming you even have room for a unitasking piece of equipment, you could buy your own preacher bench for $200 or whatever, or this for $30. An excellent alternative.
My only complaint is I think the arm padding could be a little thicker. On downward motions when I'm going slow my tricep gets engaged and the metal digs into it. It is also a little hard to find the right position for it, as far as how to adjust the strap so your arms rest on it appropriately, I had to tinker and guess because it comes with no instructions as such. It seems just above your elbow is the best spot, on the elbow itself did not work well, and well as placing it higher up the arm.