I've had the pro version for a while. You'll get out of it what you put into it. So don't expect it to be some magic wand that lets you skip real work. However it will help if you are not good at animating and just good at drawing, because you don't have to spend time drawing hundreds of frames since its all interpolated. Overall you will get smooth looking animations with high frame rates extremely quick.
As far as the pro version, I mostly got it for the inverse kinematics. Their main use is simulating how joints and stuff should properly work... however I found them extremely useful if you want to manipulate variously parts of a character at runtime. For example I set up an IK that was attached to my characters back bone, and had it change its angle based on the characters position to the mouse. This let me pivot my characters upper body to aim a weapon without having any extra animations created, it just adjusts the animations to what that IK is doing. Can also do some cool things with mixing tracks and what not. Like having legs animate running, while upper body does other animations. Which also works well with the IK thing I mentioned.
Oh and transitioning from one animation to another can be done very smoothly which is nice. Its been a while, buts its just a GML function that tells it to do the maths to interpolate between two animations. Suppose you are running and have a slide animation, instead of suddently starting on the first frame of the slide it will transition smoothly between them.