Started out just collecting all of the FF main title. Once I got down into the teens, and books were hundreds of dollars a piece just for a 4th(!) appearance of Doom, I side stepped a bit. Got all the Annuals. Then got all the other main line series (Unlimited, Unplugged, Fantastic Force, and MK4). Then went for the one shots and minis. But I was always a stickler for the main, 616 universe and that universe only.
Well, I got all those and wanted more. So then I went and got all the alternate universe stuff (Ultimate and random minis and one shots I didn't have for that reason).
Now I guess I'm back to whittling down the main series again.
Being a completionist sucks. In my head, I'm telling myself I will stop at 10, and then just get good quality reprints of 1 through 9 because realistically I will never pay 50k total for 1-5.
Own every Micronauts book. Own every ROM book.
Drew the line on Silver Surfer at vol 1. I own zero vol 1. I figured if I would have to spend hundreds per book again, I will finish my interest of FF instead.