As a DM with a player who has the Sentinel feat... I can say that it only comes up on occasion, but oftentimes for me can be quite irritating.
I think I can get my BBEG out of harm's way, when the pally then rolls up and stops him in his tracks. Which is great for the player and the party, because it does exactly what you'd want a feat to do-- create a situation that only occurs occasionally so it's still exciting when it can happen... while at the same time make the DM's face drop when he realizes what's going on. That's been PRICELESS for them. And quite probably more fun for them than whatever story might've come out of the BBEG ducking out and them having to go chasing after it. In that regard, the player really feels like he got his money worth by taking it.
It also just means that as a DM you either need to accept that occasionally a BBEG ain't going to get away... or that you have to remember Sentinel when encounter building if you really want to make sure there's a possible escape route for the enemy even if Sentinel comes into play.

It also just means that as a DM you either need to accept that occasionally a BBEG ain't going to get away... or that you have to remember Sentinel when encounter building if you really want to make sure there's a possible escape route for the enemy even if Sentinel comes into play.