Recently some African countries have withdrawn from the International Criminal Court on that basis; only the weak are prosecuted. It's inconceivable that a U.S. leader would be prosecuted, for example.
However, the GC are used by everyone against everyone, including the strong against the strong, so the strong don't escape scrutiny. I believe U.S. law requires the government to obey them. For example, the Bush administration crafted careful (and sometimes convoluted) legal arguments that their actions complied with the GC - they respected the GC enough to feel they couldn't just ignore the rules.
On the other hand: The Geneva Conventions are not followed or implemented perfectly, but neither is any law or rule. International governance is anarchy; there is no real authority; in that realm, nothing will get nearly 100% compliance.
Do the GC have a positive impact? Now militaries and governments are legally bound and their people trained to follow these rules, and accusing someone of 'violation of the Geneva Conventions' carries weight. Imagine the world without them.