I agree it's not an effective currency. But I disagree that it's an investment. I think the correct technical term is speculation.
Investment is where you find something producing value in the real world and then buy and hold a piece of that. You're expecting a long stream of returns from the value being produced. Speculation is where you are making a bet on price movements in a way where economic productivity is irrelevant.
When people buy and sell commodities and currencies, they're speculating. They don't care about utility. There's no economic value being created, so one's investment doesn't grow during the time you hold, say, gold. Instead, it's a zero-sum activity: considering all participants together, wins and losses are equal. (Well, technically, losses are larger than wins, because there are always overhead costs.)
Bitcoin is easy to mistake for an investment, because it's meant to be deflationary. (It was even easier to mistake for one during the period where it only went up.) But I don't think holding Bitcoin means you own a slice of an economically productive asset. I think it's just a bet that the commodity price will go up, in the same way you bet if you buy gold or euros.
[1] Warren Buffett has talked about this distinction a lot, over the years. E..g: http://www.businessinsider.com/warren-buffett-investors-vers...