For those who don't want to read all this and prefer a good, informative video on NaPoVoInterCo (as CPD Grey puts it):
Frankly, I've always thought this was unconstitutional. I mean, states are free to apportion their electors any way they want. But they are NOT free to enter into compacts with one another. It's expressly forbidden in Article I: Section 10: Clause 3. Any complaints that states can make some limited agreements with each other would be swept aside by the fact that they would be colluding to alter the method that the president is chosen.
Anyway I suspect the whole thing would fall apart as soon as someone cheated, which probably wouldn't take long. Also, keep an eye on the Supreme Court this summer. They've taken two cases that will determine whether a state can compel an elector to cast a vote a certain way or remove them if they vote wrong. If they declare that a state can't do that (which I strongly suspect they will), then the compact becomes a LOT more difficult to enforce.