Bitcoin users actually use fiat as their currency of choice. They use bitcoin as an investment vehicle - dollars, euros, etc. are the medium of exchange by which they either buy bitcoins on exchanges or use to buy mining equipment which generates bitcoins, the investment.
I don't, and never have. I've been using bitcoins as a medium of exchange for 4+ years now, and have bought all range of items; from hats for my avatar on TF2 to handmade jewlry on Etsy to game & ebook licenses to about 16 months straight of cell service.
How does that fit into your investment vehicle model?
Bitcoins are actually a pretty crappy investment vehicle, despite the historic returns. Like gold, there is no dividend. You're not actually investing in anything, and the scarcity that speculators have been chasing is artificial. There are simply too many substitutes for that to be a good idea long term. Bitcoins are not a commodity, so they can't be made into anything, and nothing requires them to operate. Wild swings due to speculation aside, bitcoins derive
all of their fundamental value from their (current and future) usefulness as a value transfer over distance mechanism. If you're holding onto them solely in the hopes that they will rise in exchange value, I would have to recommend that you reassess your portfolio.