I was actually going to start a thread calling out the Dads/Uncles/Grandpas to show off their plastic clampack conquering cutlery. I actually find a knife to be the ideal tool for slicing these open as scissors tend to bind. However, you must choose your equipment wisely:
1) The knife should either be fixed bladed or have a very reliable lock
2) It should have a very durable edge to handle the plastic and stay sharp.