I've hired a lot of engineers and I've seen this a number of times. In my experience once someone gets to the point of interviewing and considering another job

I've hired a lot of engineers and I've seen this a number of times. In my experience once someone gets to the point of interviewing and considering another job, well, they're on the way out. I've done the counter offer and tried to get them to stay a number of times and it just never tends to work out -- they take the offer and then within six months they are out anyway. What I always do though is try and keep a good relationship with all departing staff so if there's a critical need for knowledge that only they have, they can be brought back on a consulting basis for specific problems. That way people can leave and organizationally you're not stuck in a bind.