Once I got a Time/Design hole punch, actually two, I can't say the page format has ever been an issue for me (1990).
I put all kinds of sizes in my system.
If you fold a standard 8.5 x 11 page nearly in half (leave about 1/2 inch gap), it fits perfectly in the Time/Design system.
Agree with those who say the system isn't cheap, but I think there's value versus a PDA when looking at all-in costs over a three year period, assuming that's an average PDA replacement cycle. My own opinion, of course.