Morality Kant's CI is the best standard to use as a guiding principle of our behaviour if we already care about others. However, it is neither magic, nor flawless. It is not perfect by any reasonable standard of perfection, nor need it be; for perfection - while being fine as an aim to strive for - is an impossible criterion to meet. We're much more reasonable than that. A moral code need not be perfect to be the best one readily available as a means for immediate, long-lasting improvement.
It's not flawless...
Someone who does not care about others could use the CI as a rule of thumb and arrive at unwanted, unexpected, and harmful consequences regardless. Someone could use the CI, in some logically possible scenario, as a means to intentionally harm others(steal and wish that everybody else would as well). Isaac argued something quite similar earlier. It could be used as a means to an end. Goodness is an end in itself. A method is only as successful as it's implementation.
How much does this logically possible situation actually matter though? Not much if we also recognize that all such guiding principles of behaviour are prone to the exact same thing.
We do what is good for the sake of doing good.
Kant's CI is a fantastic starting point for helping all youth develop a greater sense of the actual part we play in our own lives including the actual influence that our thought, belief, and/or behaviour has upon the world. It couldn't be more realistic and practical. It also gives it's user the best possible chance to discover, release, and/or further perpetuate goodness. Kant's CI is quite easily taught despite Kant's unnecessarily complex taxonomy/linguistic framework.
Certainly the best moral belief can be taught to everyone first acquiring language, regardless of individual particulars.
"What if everyone acted like that?"
Easy to understand. Easy to further develop.
With simple reasonable follow-up focusing upon the obvious negative consequences, it helps to further develop critical thinking skills and it begins and/or continues to promote active conscious deliberation regarding how our actions affect/effect others. <---------That is the very heart of all that is morally relevant.
Bring it to the forefront of the child's thought at the appropriate time, and it can leave quite the good impression. Teaching children how to come to such terms promotes goodness for it helps nurture a worldview built upon how important it is to consider others as well as considering the efficacy of their own thought, belief, and/or behaviour.
It fosters goodwill.
It teaches a well-grounded sense of responsibility based upon considering how one's own behaviour effects/affects the world. It is easily understood with simple terms at first. It is also easily amenable to further nuance. After it is introduced as a way to come to terms with everyday situations and is being actively employed by someone, it can be used as the sole means for helping them to finally determine what they ought do in any actual situation that they find themselves in. It will not eliminate the possibility of being mistaken.
Not everyone likes being treated the same way. Everyone likes being respected, accepted, and valued. Thinking about everyone in such terms promotes goodness. Our honourable use of Kant's CI can help pave the way.
It provides a foundation upon which to discover, release, and/or further perpetuate goodness.
If someone already respects others it is a fantastic tool for honing the will. If someone does not already respect others, say someone has a long history of not giving a fuck what sort of harm she/he/they do to others, then Kant's CI is much less affective/effective. Certainly we all agree that that sort of behaviour is not the best.
The best possible results stem from early teaching and constant reminding. It fosters good habits of mind.
Thinking about the affects/effects that our own behaviour has upon others is the very best foundation. It is easily administered, taught, and/or otherwise initially implemented. It provides a baseline from which to judge. It supports the idea of doing what is good for it's own sake. It provides the best practical and realistic possibility for increasing happiness and livelihood while decreasing unnecessary suffering.
We're not seeking perfection. We're setting out which is the most likely to increase goodness while decrease unnecessary suffering. If everyone did this, there is no doubt that the world world be a much better place than if not. So, it also consistent.
What more could one ask for?
My vote is Kant's CI, and you've just read my off the cuff meandering.