I think this is an interesting topic. I've always found it fascinating what and how people collect paper money.
I remember when I first heard about prefix collectors. I was at a show and marvelling at their fortitude, staying power, and budget when a collector with a book of numbers came up to the table. I had to ask "are you a prefix collector?" and he acknowledged that he was. "How do you do it?" I asked (in terms of money, storage, etc) and he gave me the long story. Then he said the key was hiding his collection from his wife! I thought this was hilarious -- but could well imagine how another person couldn't relate to such an extensive collection. Most prefix collectors I've met (besides young Journey collectors) are collecting 1937 or 1954 $1.00 notes.
Personally, I can't even collect all the replacements of any one series. For one thing, I'm always constrained by a budget-- so I have many strategies involved in attaining what I wish to attain. For one: I will often get 2 of the same prefix if I can get them at a reduced price. I will then sell the extra later-- and put that money towards a prefix I've been wanting. I have bought a lot of notes that I don't really need hoping to invest in notes I wish to attain.
I don't know how others feel-- but I find Cdn money too interesting to collect just one series or one denomination. I suppose if I focussed more on one -- I would be better off. Instead-- I've decided to narrow in on $20 and lower denominations. I also try to keep it at 1935 and up.
As far as other collectors go: Some I know only collect Birds or Journeys. Others are really into birthday notes, or special numbers, while others focus more on errors. Just about everyone I know have what would be called a basic collection (one UNC of each denomination) and then they concentrate on change-overs, short prefixes, errors, radars of a certain series.