Pros: STOP kids from accessing electrical outlets
Cons: stop adults from accessing the outlet too...
There's no question- these are very good at plugging electrical outlets, and I can't imagine the kid who could get these things out. The problem is that they also are very difficult for adults to remove and use the outlet.
These are better at plugging an outlet than the "old style" plastic plugs, but at a price - adult usability. In order to remove these plugs, 3 buttons need to be depressed (pretty tough buttons, too) and the plug needs to be pulled out at the same time. It sounds easy, but is actually fairly difficult because the buttons are so small and require so much pressure.
I have no concerns about my 8-month old being able to pull these things out, even as she gets older - because my wife can barely pull them out!
That said, these have a very good use in my house - all the outlets that rarely or never get used (you know, the ones behind the couch or on the bare wall?) get a pair of these on them, because they are so good at stopping access. The outlets that get regular use have a different type of outlet cover - which I wrote a review on HERE"
They are cheaper than the sliding outlet covers, and more expensive than the old style plastic plugs. I just wouldn't use them on outlets that required frequent use.