Counter offers are career poison. As a recruiter it's paramount to know that. Even more important is how well potential hires know this. It's common for any good employee to get a counter offer when they leave their employer. No employer wants to lose a good resource or intellectual property and odds are they will offer a raise or promotion to prevent it. Odds are just as high that it won't fix the problem. Go ahead and google "Counter Offers". The articles are out there in spades, like this clip from TopCareers.
According to national surveys of employees that accept counter-offers, 50-80 percent voluntarily leave their employer within six months of accepting the counter-offer because of unkept promises. The majority of the balance of employees that accept counter-offers involuntarily leave their current employers within twelve months of accepting the counter-offer (terminated, fired, laid off, etc.).
Recruiters do themselves and their recruits a disservice if this isn't made very clear from the start. The worst thing in the world is to have a recruit accept a counter offer, then call back three months later to look again. Chances are they've burned a bridge and lost an opportunity.
