The ESRB is only as good as the store AND parent that enforces it.
Personally I think it is a good idea, and one of the few times that I'm okay that government is involved. But I couple that only by saying the ESRB is 'suggesting' not 'enforcing' morality.
The ESRB in no diffrent from television or movie ratings. They are there to serve as a guideline and warning flag to parents. Seeing that most children need money from their parents to buy video games, parents are directly/indirectly involved in the transfer of cash to the store.
If a company like Rock Star puts a M+ sticker on their game....as far as I am concerned, they've done their job and due dilligence. If a parent go ahead and buys their 13 year old a game that drop the F-bomb and simulates prostitution pick up and sex acts in a parked car...I don't blame Rock Star. The parent is ignorant.
The store are just as bad. You may have 1 employee out of 3 or more that actually card the kids. We already do it for tobacco and alcohol, so this shouldn't be a foreign concept to game store employees. I've personally witnessed young kids buying mature titles in front of me with the sales associate just as ignorant.
Now if the government stepped in and used a little muscle in the terms of fines or jail time to store owners, then perhaps employees and gamestores may take it more seriously.
The argument that there are too many games to police is ludicrous, or even more so when parents feign knowledge on the subject. With today's internet, blogs, magazine, newswires talking about games, how can parents (BTW- I am one) play stupid? When I take my kid to the store, the rating is right there on the front of the package...how can I miss that? How can the employee miss it? How could I miss the advent of GTA IV with so much advertising and hype before it even came out. I own GTA IV myself, and enjoy it immensly....but no way do I play in front of my daughter.