I've used the cheap repair kits quite a few times, most recently just a couple of weeks ago. They work great, never had a problem. Even though they should be only used for a temporary repair, I have pushed my luck with them and never had a failure of the plug. I do have the full size spare as a backup still, but I can plug the tire almost as fast as I can change the spare.

Even though I've never experienced a problem, I've been meaning to upgrade to a better kit, one with metal handles. Somewhere I saw pictures of what happens when one of the cheap T-handled plug reamers break while you're trying to push it through the tire, and it wasn't a pretty sight. The plastic probably gets brittle from old age and heat, so when I use it I try and be careful about how much force I apply. The next one I pick up is definitely going to have metal handles.

I'm guessing the reason they don't recommend it for sidewalls because the sidewall is subject to a lot of flexing, so a plug probably wouldn't last very long there. How long is too long, I don't know, but I would not hesitate to use it for a sidewall puncture if that meant getting home. At worst, it fails down the road somewhere and you either plug it again or you're stuck further along. Although, I'd probably start looking for a replacement as soon as I got back home.