I personally don't like them and would rather put in real shutoff valves, but my guess from a DIY viewpoint is it's okay. The seal on those things is a little bit of rubber. It pierces the copper to let the water in. On most of them I've read the instructions for say not to use them as a volume control, but they do (usually) let you shut it off. If it gets knocked and you turn it off, it can make the hole larger when you screw it back in. Not usually a problem.
I've seen a 3-way shutoff valve in a catalog. Replace the existing valve with a 3-way, and no saddle valves needed.