My take is that bitcoin isn't currency, but shares some aspects with it. In the end, it doesn't really matter whether or not bitcoin fits an established definition, or even if it can be well defined. The most important question is whether or not people find it to be useful.

As a store of wealth, it very well may be. I have BTC, and no one can take them from me. However, I can transfer them instantly to anyone at a very low cost. That is useful. Also, I know that only 21M BTC will ever exist. That is also useful to me. A new large deposit of bitcoin cannot be discovered. In some ways, BTC looks superior to gold as a proxy of wealth. Gold has some uses, but I believe 40% of its current value is due to its use as a store of wealth. All proxies for wealth employ illusion. These illusions can be dispelled, but certain factors make this difficult. The open question is whether or not bitcoin's illusion can be dispelled. Personally, I think that every day it becomes less likely that it can be.

I do think that it will remain to be volatile for some time to come, however. The market cap for bitcoin is currently $3.6B. A rough amount of privately held gold in trading funds somewhere around $100B. If you add gold reserves of countries, it's a couple of orders of magnitudes higher. In short, there's plenty of wealth that could be transferred into bitcoin if it is going to stick around and people really buy into the illusion. On top of that, about 9M more BTC have yet to be mined over the next 17 years or so, and the next halving of the mining reward isn't until 2017.