The intent is to make it behave as close to the left analog stick of an xbox 360 controller as possible. You can reach any X,Y coordinate that the 360 controller can. However, the range of motion with Aimpad is greater because the xbox 360 controller never reaches the corners of the Axis as the shape of the joystick device is a circle whereas the X,Y axis that the movement is plotted on is a square. You can try this out for yourself if you attach a 360 controller and go to Game Controllers in windows and select the properties of the device. You can move the left analog stick and you'll notice that you can't reach the corners of the X, Y axis. Essentially what this means is that when you move diagonally you aren't moving at the full speed you could be. Aimpad actually uses the full range. Depending on the game the effect is subtle, but the difference does exist.
The other unique thing about Aimpad is that while you can move your trajectory in 360 degrees based off how far down you are pressing each key, you also can VERY easily move exactly in 4 directions at any speed you want. This is useful because often times you want to run precisely forward but occasionally if you aren't pressing exactly up on the xbox 360 controller you will shift to one side or the other. A game that effectively demonstrates this is Darksiders. Many times if you aren't pressing exactly straight up (or pretty close) you find yourself moving in a slightly off center direction. I found it annoying, but that sort of thing may not bother you. However, in another game "Deadlight" (a side-scroller) I died MANY times unnecessarily in that game using the 360 controller because I wanted to jump straight up. But I was not exactly holding the stick straight up so when I jumped I would jump off a cliff into less than friendly zombies who promptly ate my brains.
That being said, controlling speed in the standard 8 directions is very simple with Aimpad and has a very low learning curve. But, adjusting to pressing two keys at slightly different depths (especially when 25 years of PC gaming has told me to mash those keys down all the way) does require more practice. I'm very good at it now, but it did take a few days to retrain my brain and fingers.
Hope that answers your question!