I have not fallen for the number on the spine thing with the Criterion Collection thankfully. Not only would it be quite costly, but they put out a lot of crap.
I did fall for it with Fox Studio Classics and Fox Film Noir. And the Warner film noir boxes. I'm happy that I did. There were a lot of great films I wouldn't have seen if I hadn't.
I love buying box sets. The only series that I can think of where I didn't buy all of the movies would be the Love Bug series. I have The Love Bug and Herbie Rides Again. But I skipped the others.
One thing that I get sucked into is collecting all of the movies for an actor or director.