Actually, it can get a bit hard, be sure to rotate your heat sink before you try to pull it free, you just have to twist it a little to break the AS up. People have damaged CPUs by just trying to pull the heatsink straight up off the CPU.
with a amd cpu it seemed to cure really hard and held the heatsink on like glue. now i'm using intel and it doesn't seem to cure the same way. i'm not sure if its the different heatsinks or the two different tubes of AS5.