I have a pair that I have used for a few years for skiing here in NE. They help, but are certainly not anything like an active heater. I have had no problems with them interfering with buckles or bindings, but if there is an interference they're flexible enough that you could pull the front of the toe up enough to remove completely from the boot-binding interface. I also try to do minimal walking in them, as the strap runs under the sole of the boot and will wear quickly if you aren't careful.
I ordered a pair for my son this season, as his boots leave him no room for warmers, and he does a lot of night skiing. Plus early season this year was so cold...
Definitely worth the low cost, but if you're really struggling with cold toes this won't solve all your problems.