I don't think the BTC will ever be a 'currency' as the USD or the GBP. It is also not necessary for the BTC to be at a 'fixed value' at any time. As a virtual currency it's ok for the BTC to fluctuate heavily as the only thing needed for it to still succeed is that payment systems are 'always on' and can determine a spot price for goods and services.
I would rather see the BTC tied to 'Pizza index' than USD. This would make it easy for people to use an in-app value determinator when making purchases.
Say you just tap your phone with a BTC wallet against a cash register. The register answers, well your burger will be 0.00065 BTC at spot price as of 12.24.44.32 PM. Accept/decline.
That's it. I see the BTC/LTC (Litecoin) as alternative currencies but still worthy of a HUGE market share for micro/everyday transactions.