It won't stop anyone who has the skill to author cheats themselves (i.e. someone with basic reversing and programming experience) but it does do a good job of detecting and banning players who are using widely distributed "commercial" cheats.

I look at EAC as being very analogous to a virus scanner, very easy to bypass if you know what you're doing, but good at catching the common and soon-to-be common threats. It also puts up enough roadblocks to make some novice attempts at cheating inconvenient at least.

Source: I deal with all the anti-cheat stuff (including EAC) on a semi-popular multi-player game. I can verify that it does make a very tangible difference to our player base in terms of the number of cheaters they are exposed to on a per match or per session basis.