Great news: The federal government is now investigating this case. Hopefully this can lead to Zimmerman being brought to justice. The feds don't have a great record when it comes to protecting the civil rights of minorities but they have a much better one than a state like Florida.
Joe Horn case for an example.
Florida has a very extreme version of this law that extends the law to public spaces: If someone steps to you on the sidewalk, you are legally entitled to defend yourself, and if there is no evidence to the contrary it is assumed to be self-defense. The local police have at times (important to note they've given multiple stories and multiple justifications for not arresting and charging Zimmerman) said that their hands are tied in this case due to the Stand Your Ground law: Since there were no direct witnesses to the shooting, it is assumed that Zimmerman is acting in self-defense automatically, and the police have their hands tied on arresting and charging Zimmerman due to the law.
I think the law definitely helps to protect Zimmerman, but it is not completely protecting him from prosecution in this case for a few reasons:
1) Zimmerman followed the victim and then left the vehicle and provoked a confrontation, instigating the situation. If anything Trayvon would have been within his legal rights to kill Zimmerman in self-defense, but not the other way around. Additionally, Zimmerman disobeyed an explicit order from the 911 dispatcher to let the police handle the situation.
2) There are numerous witness accounts that dispute Zimmerman's version of events enough that an arrest and charge would be warranted under the law: Numerous people heard Trayvon screaming for help, and his screams can even be heard on 911 calls.
So while there's some talk about the laws here protecting Zimmerman I think it is a very serious misreading of the laws on the books. The laws are quite extreme and probably dangerous but in this particular case it looks like a corrupt and racist police department probably has more to do with the man not even being charged than the law does. There's more than enough shadiness to this case to make the self-defense argument tenuous enough that the man should be arrested and charged and be forced to argue his case in court - again, hopefully now that the feds are involved this will happen.