Really good
Overall, this is a wonderful addition to the Portal. The gameplay is solid and the objective is clear (and usually manageable). However, I've noticed that there are occasionally problems. For instance, on level 14 it is possible to throw your dart straight through the right side of the "maze." I'm not sure if this happens because it hits the wall at a certain angle or speed or something else entirely, but it created quite a bit of frustration for me.