I have at least one of those in one one of my tool kits. They work well enough on PVC pipe. But a simple length of nylon mason's line will also work. It is an old plumbers trick. Works really well and keeps you from having to lay down in the mud and have at a run of pipe in the bottom of a hole. In both cases they work by friction and melt their way through the PVC.

There are flexible wire saws available. Coughlan's makes a cheap version that sells for about $5. They are good for one cut unless your very careful.

There are more expensive ones that are usable. Best Glide sells a nice one that hold up much better than the discount store version.