I almost never buy a Filofax until I actually need it. Or, more accurately, until I immediately need it!
When I first came back to Filofax after failing to organize my work life electronically, I darted into Bob Slate Stationers in Cambridge (the one in Massachuetts) at 5:55 p.m. (they close at 6:00) to buy the only A5 they had -- a boring black Finsbury!
That said, I now have 5 A5 Filos, 2 of which I've never used, even though I did have a use in mind when I purchased them. I own 4 Personals, all of which are well-used and becoming more so!
Do you ever buy a Filo with no plans for its use, just because you love it and may use it someday? Or are you collecting them instead of using them?